I beleive all the .12/.15 engines have the same mounting dimensions, and all you need to look for is the crankshaft length (Associated uses short crank, but a long one can be cut off for use in Associated cars). Also if you want recoil start or not, as the cars equipped w/ recoil may use taller engine mounts for clearance and a different header pipe. If you change engines, it will be easier if you stay with the same style carb that you have (rotary vs. slide) for ease of hook up. If you disassemble your engine and clean it out, replace bearings, you should be able to use it with no problem. I hate the idea of buying a used engine, as you usually are getting someone elses problems/junk. Keep an eye out for sales on new engines, I see good prices advertised in R/C car action magazine and at Tower (go to their sale section of website). Good luck.

BTW, to avoid more rust in the future, start using an after run oil.