What interchanges with an 84 kdx 200 cdi box

Yes, it can be fixed because the company only changes the design, not the capacity and size.

bad brain box/cdi, did it cut out after it got hot?

Is battery voltage up to at least 12.5 volts? Have you checked ALL of the ground points related to ignition? Is there spark at the spark plugs when engine quits? When running, does the charge show at least 13.5 volts on battery? Have you had the CDI unit checked at a Yamaha dealer?

THIS QUESTION DONT MAKE SENSE SINCE THREES ONLY ONE SPARK PLUG ON A KDX 125..BUT A WEAK SPARK COULD MEAN ANYTHING IN THE ELECTRICAL SYSTEM..CDI,STATER,WIRING LOOM,CORRODED CONNECTION SOMEWHERE...ITS HARD TO SAY WITHOUT SEEING IT..AND SYSTEMATICALLY TESTING ITS COMPONENTS..

The Internet or junk yards.. Thats where i find an get all my parts

Make sure you have recommended spark plug. 2cycle bikes require a hot plug in order to burn fuel efficiently. I'm not that familiar with your bike but it seems too new-ish for the rings to be bad already. Another thing is... that you can't just use any 2cycle oil... you must use the manufacture's recommended oil for the fuel. Many of these machines are picky about which oil is used. Fuel to air mixture many times is the problem... this has to be just right on a 2cycle... they are touchy if they don't get the right amont of air. I hope somehting here will help you to find the answer or at least eliminate a few things if nothing else :)

Try following the wire from the stop button on the handle bar back to the CDI box. Simply unplug that wire. CDI is disabled by grounding. Does the KDX have a key switch or just the stop button. Try just connecting the pick up wire to the CDI and CDI to coil. You have to isolate ground to CDI to run. Check air gap at pick up. Use a matchbook for a gauge.(.018)

In order to better help you with the bike, more info is needed... does it have good compression? does it have a good spark plug? is it getting good spark? is the air fliter clean? is the carb clean? is fuel getting into the carb? is fuel getting into the cylinder? after kicking several times can you smell gas as if it were flooding? did you kick it over a few times and pull the plug out (QUICKLY) to see if its wet with fuel? does it look like there is any condensation on the plug after kicking it over several times? go thru all this info, and please let me know what you find and if this helps you.

I believe so. Go to a couple sites to verify part numbers.
Try www.bikebandit.com

The side stand kill switch can go bad and do this. Check the switch with an Ohm meter. Also, check the wiring harness connections. Fix any loose connections. Check the handlebar kill switch. It can go bad also.

Please rate this answer. Thanks hd87lovetori!