I have a 80 yamaha qt50. It runs, but very poorly. I took off the exauhst pipe and found the piston has scratches all over it. It is probably not getting proper compression because of this. What can be done to fix this? Can I just buy new rings or should I get the engine bored larger and get a larger piston? I dont want to spend much money because this thing is pretty old, I just want it to run ok. Any help would be great.