_i’ve removed the exhaust bolt to see if the exhaust could cause too much back presure_
You mean the big bolt in the side of the muffler? That bolt only allows you to go in and clear a small tube. If that tube is clogged, removing the bolt will do nothing... the bolt is on the outer, clogged-side of that tube. Stick a screwdriver in there and clear the tube.
All that heat means friction .. you were asking about new belts earlier too.
My guess is the belt is slipping for some reason, or that the pulleys are not operating correctly. When the front and rear variator units do not work together, but fight against eachother, lots of friction can be developed.
For instance, if the rear unit's inner face were rusted on it's shaft (not really a shaft) it would not open wide easily.. that could cause belt slipping and cause the belt to pull a lot harder than it needs to pull.
And if it wasn't mechanically able to open all the way the front pulley would be keeping the belt extremely tight at high speeds... more friction.
Mine get nice and warm but nothing like hot.. no sense that it would burn me .. Sometimes i notice it's quite cool after a ride. I think it depends on how you're riding.. steady speed or lots of speed changes.. someday i'll check it out.