The gas caps should interchange. As fas as I know, most all 40/50 stuff swaps around fine, over many years of production.
It's not unusual at all to have a 2-stroke spit mixture out of the intake; it's pretty darn common, actually. Having the correct airbox will help this to some extent. And yes, if the bike was tipped over it could easily have dumped liquid fuel out of the carb.
As for the gasket, I think you are correct - put the split face against whatever has the largest and flattest surface. In this case, that should be the cylinder. Not 100% sure about this, though.