Bought 3 yr.old MPV4 with dead batteries and flat tires. Can I put foam filled tires on the current rims, do it myself, and use the same rims? What size are the tires? where can I get a manual without paying $18 + shipping?

https://www.youtube.com/watch?v=58STtUM-Wow

The pioneer 3 S131 tire size is :
3.00 - 4 or
260 x 85 or
10" x 3"
The front foam filled tire is available from New Solutions 800-429-9600.
It is probably the ribbed rounded style tire # F057. This profile makes it easier to steer. I am attaching the catalog pictures to match the numbers on your existing tire to their product line. I would also recommend replacing both rear pneumatic tires with foam filled. Hope this helps.

You will need Socket wrenches, a big ***** Driver and air valve tool if there is a pneumatic tire involved, two 3"-5/16" bolts w/nuts and washers. I like to tip the chair on its side. and loosen the main shaft nut and remove it. Then wiggle the tire back and forth, then lift. If you are lucky it will slide right off. then you can let the air out of your inner tube or go straight to removing the split rim bolts. the ***** driver is used between the rim and tire and pry apart the tire from the rim, work around the the rim until the tire is off on both sides, If this is a foam filled tire then you will need 2 5/16" Bolts w/ nuts and washers replace the tire and line up the hole for the bolts and push the rims together so the long bolts can make it long enough to put the nut on. the nut down a little then put another one on opposite side to pull the wheel in tighten the bolts a little on each side evenly until the original bolts are able to be used to tighten then take out long bolts and install all the rest of the shorter bolts. tighten them up and grease the shaft and put the shaft key in the slot and gently put the wheel on and tighten the shaft nut. Check to see the wheel turns with the shaft. the key can slide out some times Then tip the chair up and see how it drives. Sorry no pictures yet.

I’m happy to assist further over the phone at https://www.6ya.com/expert/bill_bcbb89783b08c489

They are most likely loosing air at the seal at the rims. That happened to me. I bought some inner tubes and problem solved.

air it up and spray it down with mild soap and water mixture (gallon of water with a teaspoon of dish soap mixed in). where it bubbles is your leak. spray whole tire (tread and sidewall), the edge of wheel where tire bead is seated to it and the valve stem. If there is one or 2 leaks on tread, you can plug with automotive tire plugs. More than that and you should buy a tube.

Dirt bike tires are pretty easy to change yourself and all you need is to remove the tire for the bike and have at least 3 long flat head screwdrivers that are about 18" long each. You'll save yourself a couple bucks doing it yourself and get the satisfaction of doing it yourself. But any MC shop will also do it for you at a cost unless you're buing a new tire there, then they wrap the tire change into the cost of new tire you bought. But if you decide to do it yourself, here's a couple tips. There are plenty of videos out there on how to do this so here are some quick pointers.
1) let any remaining are out of the tire.
2) for a dirtbike, loosen the rim locks. (these are on some dirtbikes and you'll know if your bike has them because there will be a bolt sticking through the rim pointing towards the center of the tire. You'll have to loosen the nut on the rim lock bolt to loosen the rim lock.)
3) with all air out of the tire and the rim locks loosened, stick your first screw driver between the lip of the rim and the tire and pull back on the screw driver.
4) When you have pulled back on the screwdriver, you'll see a gap where you can then stick the next screw driver.
5) Don't try to lift too much of the tire at the beginning. there are steel bands in the tire and you don't want to snap them or destroy the rubber that surrounds the steel bands. Because if you do mess it up too much, the steel band is made up of a bunch of steel wires, and if you snap those wires, one could come through and puncture you tube when you put it back on and fill it with air. So don't take too much tire off at one time. You'll get the hand of it.
6) With one side of the tire out you can then reach inside and pull out the tube. Fill it with a enough air to expose the lead and then you can decide to replace the tube or patch it.
7) After repairing/replacing the tube, put the tire back on in the reverse order.
8) Don't inflate the tire all the way the first time. Inflate it half way and bounce the tire to get the tube to sit right.
9) Tighten the Rim locks (if you had them)
10) inflate the tire to the proper pressure.
11) Put the tire back on.

It should have innertubes like a bicycle. Just take the wheel off and use a flathead screwdriver to pry one side of the tire off the rim. Be careful to not damage the tube when doing this. Once u have one side of the tire over the rim you can pull the tube out. You can use any bicycle tube repair kit to fix any leaks you find. Easiest way for small tubes like this is just fill a sink with water, air up the tube and put it under the water to find any leaks.

Get the rim / tire of the frame. Let all the air out of the tube, Using two flat blade screw drivers, put them between the rim and tire about 3 inches apart, (be careful not to puncture the tube) Pry the tire over the rim, leave one in place and move the other about 3 more inches and pry the tire over the rim again, keep moving around the rim until the side is off. Reach in and pull the tube out, fish the stem through the hole. Finish pulling the tire off the rim, you can use liquid soap to help make things easy. Put the rim back together the same way. Hope this helps Fix Ya up.

Are you planning to buy it online or at a store? Just a general price?

I assume you are referring to the caster tire, not the drive tire. You will need to check to see if the rim is a split-rim or a solid rim. A split-rim will have 4 or more bolts holding the two halves of the rim together. If you have flat-free tires i.e., foam-filled solid tires, evidenced by the lack of a valve stem which is used to inflate non-flat-free tires (pneumatic), you would remove the wheel from the caster by rermoving the center bolt acting as the axle. Then remove the bolts holding the rims together. Reposition tire on top of one of the rim halves, then place the other rim half on top so that the bolt holes in the top one line up with the holes in the bottom one. You need to buy a couple of bolts that are about 3/4 in or more longer than the bolts that you removed, as you now must press the two halves back together and the stock bolts probably won't reach through. Put the two longer bolts in opposite of each other in two holes and tighten down. When the rims are pressed together, then put the stock bolts in, remove the two longer ones and replace with stock bolts. Place wheel back on the caster. If you have pneumatic tires (requiring air), make sure to always deflate the tire completely before attempting to loosen the bolts holding the rim-halves together. The same process is used for drive tire replacement. If the rims are not split rims, then it is easiest to buy an entire new wheel and tire as getting it back on is quite a chore requiring a press and lots of tequila! It's easy to break the rims. Good luck.