Motorcycle Repair: Replacement motor for 1973 Yamaha TX500, yamaha tx500, simple test

Question
I recently bought a 1973 Yamaha TX 500 that looks great but has engine problems. After resetting the timing, rebuilding the carbs and replacing the clutch (the 3 most likely causes for my lack of power) I took it to a mechanic. After he spent quite a few hours on it he said he couldn't figure it out either and felt as though it would be a better suggestion to find a replacement motor for it than pay him to spend more time without any guarantees he would be able to fix it.

My question is this... I am not impressed with the TX500 engine  in general.  They seem unreliable and prone to problems. I would rather a different model in the bike.  What other engines will fit that frame (Yamaha or other), and if you aren't sure could you point me in some direction?  Thank you!

Kurt

Answer
Kurt, Just do a simple test for your own. Check the compression. If its low this may help you decice if its worth continueing do what is called a dry test and hold the throttle open. Then add about 3 squirts of oil to the cylinder and retest it wet. If the pressure goes up a lot it the rings not sealing well untill you put oil in it to seal off the rings. If it doesnt go up much if any its the valves. They could be burnt or bent or the adjustment could be to tight and may need just to have the valve lash reset. As far as a different motor to fit this frame Im not sure. Sorry.....