Are you sure its the motor?
The hydrostatic pump is devided into 2 seperate sections, one for left one for right. One section could be bad and replacing the motor will not fix it.
Some motors are not joined inside the chain case, and even the ones that are i don't believe they are directally connected to the chain anyway, they go thorugh a gearbox. Try removing the 3 hoses and the 4 large bolts and see it the motor will slide out. I would then try swapping motors and see if the problem moves to the other side, if it does its the motor if not its the pump.
Both pumps and motors are still available for this machine. A fully re-built pump is 2,300 including a refundable $500 core charge and the motors would be around the 500 mark. If you have some mechanical ability you should be able to repair the pump for almost nothing if its just wear on the swash plates. If i remember i will post a link for these, i don't have it available to me at work.
I have done a fair bit or work on 743s that are essentially the same, they use the same pump and motors. You can contact me direct if you wish, my e-mail is in my user profile.
With propper care the machine will last you many years.