20% oil isn't very common. Most fuels come through with 18%, and although 16% isn't great, it won't hurt anything as long as you run the engine on the rich side.

Do NOT remove the head unless you have to rebuild the engine or something. When you put it bak on, did you tighten the bolts in the proper order? You can warp the head if you don't.

snug one down about finger tight, then do the same to the "opposite" bolt. The move to one of the loose ones and do that, then the last loose one. Start back at the beginning and tighten the first one some more, and repeat the order you folloed the first time around.

Don't overtighten them either, it's not too hard to strip the threads.

I'm guessing that someone with experience would get that engine running pretty easily too. It's pretty hard to damage them to the point where they won't run.

Good luck,
Dennis-