You should be able to go to the Sears replacement parts web site, enter the model and serial number and get several pages of parts break down illustrations.
Think I would get the engine running first, then decide if it is worthy of investing in the drive train.
BTW, you have the best mower ever! A free one! I've never in my adult life bought a mower, new or used. All have been free, thrown away because of some minor problem.