RE: problem with my GMS 1.20
check the fuel system. Chances are your crash screwed up something inside the tank. Cut clunk line or folded clunk line.
Any time you have an engine that is running well, then suddenly is not after an incident - check the fuel system completely.
Stick with the F plug. OS#8 is too cold for that engine.
Depending on your crash, you may have gotten dirt in the front bearing, and if it hit hard enough, you may have a bent crankshaft.
Check the fuel system first.