the ring gear on the fly wheel may have damaged teeth or not have the right gear teeth count to be compatible . Pinion teeth have to match the ring gear teeth or they have trouble meshing and that makes it hard to turn over. If the ring gear teeth have been damaged then the gear box will have to come out to get the fly wheel / flex plate off. If it is auto then it is a simple matter of replacing the flex plate . But if manual the ring gear is removed from the flywheel and a new one fitted. Most accredited service shops can do this operation. count the number of teeth on the ring gear and the number on the starter pinion and take the figures to an auto electrician and he will know it they are right.
Most ring gears are only shrunk fitted on the fly wheels. REmoval is by simply driving a good cold chisel in between the teeth and cracking the ring . Replacement is by heating the ringear evenly and placing onto the flywheel and letting it cool down. Ensure that any timing marks on the ring gear are in exactly the same place as the one removed. THis heating process may not work if there are sensors in the ring gear for cpu operation Check with service dealer for further info.
Thats your starter ring that you need to remove its a press fit so start by supporting the flywheel with some wooden blocks and then heat your starter ring with a torch so it will expand and can then be driven off with a hammer and drift punch.
To install the new starter ring you will need to use your wifes oven set at 450 degrees to heat the new starter ring for about 20 mins so it will expand and put the flywheel in the fridge at the same time so it will contract, have your hammer and drift punch ready so you can seat the starter ring on the flywheel before it cools down. P.S. Use oven mitts so you won't get burned!
You need to pull the tranny out. Then the flywheel needs to be unbolted from the engine. if it's an automatic, purchase a new flywheel, if its a clutch, you can have a new ring gear pressed on to the flywheel...
You have a timing chain or gear to gear (no chain) ? Can't see why the top gear wont come off (if it's a chain setup) Bottom gear can be split and removed with a good chisel. (tap new one on with a pipe around crank using a hammer) Usually easier to split where woodruff key is.If gear to gear, you need to heat the center with a torch.(the gear, not the cam or crank.)