Looking at the piston and sleeve, when it is visibly scratched, you may be approaching rebuild time. Assemble then engine and put the glow plug in. Turn the flywheel. It should be difficult to turn it past tdc if there is good compression. If you can turn the flywheel easily with one finger all the way around, it's rebuild time. Of course, if the engine is still running well, don't bother rebuilding it.

Clearest symptom of an engine that is approaching rebuild time: Engine will start and run fine, until it warms up to temp. Then it dies and will not start again until cooled. If this is your motor, spin the flywheel while the engine is still warm (don't burn yourself). If it spins freely with no compression at tdc, it's probably rebuild time.