I just opened up my O.S 46AX to clean it as I run it on pure castor and methanol so it gets pretty gunky. I discovered that the piston and piston liner are clearly marked by elements that have been through the cylinder and I don't know up to which point are these damages tolerable and when should I think about replacing both parts. Doing a test with a tacho is enough to know if the engine is losing its power or should I seek other pointers?