It's an AIR COOLED engine. Continuous riding at WOT will heat up any engine, especially one that has no other means of cooling except moving air. Increasing compression only makes it worse.
You need to remember that it is a motorized bicycle, not a motorcycle. It is not intended to ride all day at top speed. It also sounds as if you're running a little lean. Richen up the mixture a little (bigger main jet) and then give your engine a break by backing off the throttle. You might be surprised how much better it will perform and how much longer it will last without abusing it.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"