The motor is failing on high rpm

I had a problem running my carb at an angle far less extreme than yours, and the discoloration on your head looks like a sign of a massive leak... but it's a touch odd because it's usually from the head gasket so that "stain" would be lower - perhaps yer leaking from around the spark plug and/or headbolts?

The thing on the carb is usually referred to as a "primer" - sometimes even a "tickler" BTW and don't worry about air leaking there - that wont effect anything, but it combined with the radical angle of yer carb probably IS causing the puddle on the floor

4 stroking is usually a sign of runnin' rich, but I'd blame a combo of tilted carb and air leaks causin' yer heartache - at least from that pic anyway...

looks like they're toasted. the dark carbon deposits on the top of the piston are normal. but the sides and the inside of the cylinder itself should be nice and shiny clean.

looks like your cylinder walls are scratched and scored. this is allowing the burnt fuel to blow past the rings, creating those brown, scorched marks.

that's not good.

did you say you put in new piston rings? they're probably the wrong size.

and when you said you "polished" the piston walls, what did you use? did you leave any abrasive residue in there? did you scratch or gouge them while re-assembling?

looks like you're gonna need a new motor.

and your carb is on way too harsh of an angle. it should be as level as possible, and the way yours is, it's like pouring the fuel out of the float bowl into the motor.

as far as the discoloration on the top, mine does that. oil leaks out through the spark plug after a hard ride, but it doesn't seem to affect mine.

looks like you're a victim of the "if it ain't broke, don't fix it" syndrome.

also, that last picture of your cylinder wall, that dark spot in the upper right hand corner looks like a head gasket leak, and it looks like there's a chip in the block right there.

also, not good.

It looks rough but you can run it till it dies I would try to fix the intake to a more level setting
if you can you probably need to get a stock intake or make one. I know money is hard to come by these days so sometimes you have to make do. I'd clean the piston and cylinder with carb cleaner get it as clean as you can do not use sand paper or try to hone the cylinder. some of the brown won't hurt. You might try running come castor oil for lube do get the castor oil made for engine use mix it at 32 to 1 use regular gas. You might try to lap the head and top of the cylinder with wet dry paper glued to a sheet of glass to get those surfaces flat. Use some cheap aluminum spray paint on the head gasket both sides install it wet or a little damp then torque the head down.
I've experimented with a very light skin of silicone rtv on both sides of the base gasket and the intake gasket that seems to help seal those surfaces pretty well. For the seal between the intake and carb a light coat of silicone there will also help seal that area.
I use never seize on the spark plug just a tiny dab on the threads that will sometimes help the plug to seal or replace the plug .
I try not to use the acorn nuts they can bottom out on the stud and not allow the nut to tighten on the fitting I see you have them on the intake flange.
I finished up on an engine I bought on ebay today I took the engine completely apart split the cases and glad I did I found a big amount of metal filing inside the engine along with other trash(this engine hadn't even been test ran at the factory luckily) if I would have mounted the engine and ran it it would not of lasted very long. this engine also had the acorn nut on the intake and head long story short those acorn nuts were bottomed out and the head gasket would have leaked and same with the intake manifold.I ended up doing quite a bit of work on the engine started it up tonight and I think its a keeper runs very well now I have to get it broke in and I'm using castor oil for the break in.

I think your saying a cylinder stud hole has striped out threads. You can get a repair kit called a ( heli coil this is a brand name) or a thread repair insert to repair damaged threads check at a auto parts store. The instructions are with the thread repair kit. You really need to fix the cylinder studs having only 3 will soon cause a leak and other problems

thread repair kits go by different names you can google thread repair and see more.

Are those new rings the same thickness as the old rings? If they are too thin for the piston grooves, that can't be good.

Plus, wouldn't new rings in an old cylinder take some wear-in time for max compression (and top end)?