they arent too bad remove the seat, and you can see them bolted to both sides of the chain case tazza knows more about the seals than me if you need I can help with some scans of the service manual I have a 630 Good luck, Jeff
That is a real bad idea, the chaincase can get water inside it pretty easy and runs chains and sprockets so you get metal shavings. The oil in here is not filtered either, please never use oil out of there in your hydraulic/hydrostatic system. Removing the motors is fairly simple, you may need to remove a hose or 2 off the motor and un-do the 4 bolts. Simply pull the motor ourwards and it will come free. There is an O ring inside a bronze bush on the end of the motor and one large one on the outside of the motor. The inside one is the critical one, the outer one has no real pressure on it but it seals the chain case.
Best to remove the wheels on that side of the machine and block it to give better access.
This won't fix your drive problem but it will fix the oil leaking to the chain case.
There is a hidden filter that hopefully is the cause of your lack of drive. Remove the seat and look at the top of the hydraulic pump, there should be a square block. 3 hoses enter the left side, remove the middle one and the nut that goes into the block, its a larger nut than the other 2. Behind here is a bronze filter. Replace or clean it and *hopefully* your problem will be fixed.
It would be a good idea to change your hydraulic oil and filter too, with all that oil from the chaincase you may have water contamination.
If anything is un-clear just let me know!!!