If the damage is as bad as I am guessing based on oily plug and loss of idle, I'm gonna probably tear the engine right apart and do the oil gallery plug and everything else I didn't get to, maybe some fresh paint..
Well, you still have a couple of months of crap weather until "real" riding season starts. Did you see any metal filings on your oil screen that would indicate that the plug is backing out? I never did on mine in many years of use. It's a lot bigger job than swapping pistons but if you're worried about sand in the bottom end I guess it's gotta' get done.
What a mess.