What are the most noticeable mods for this kind of engine? What has worked for anyone?
Inexpensive mods.
1) If it has 2 head gaskets remove one. The thin aftermarket helps also.
2) Change both spark plug, wire and boot. The wire unscrews from the CDI. Seal wire to CDI (weatherproof and secures it).
3) I made my own boost bottle and can tell a difference, some riders can't.
4) for a easier pull on the clutch handle I've installed a roller bracket that the cable runs through. Not my idea but I make them and sell them. Unbelievable difference in clutch operation. Also some grind (round it off) the arm that pushes the clutch pin. Factory is squared off.
5) I've heard that the exhaust expansion chamber makes a difference...I don't know my bike runs both fast enough and fine with me.
6) my next mod will be a shift kit, so I can run the same speed at a lower RPM...save engine life.
7) remove and replace all Chinese hardware with grade 8.8 (same as our grade 5). Use locking nuts on ALL studs.
8) I use loc tite (red) on all studs followed with self locking nylon nuts. My head torque is as good as it was a year ago when built. Have not had to re-torque, checked them yesterday.
9) proper torque, use a inch pound torque wrench if able. Do not guess on torque. I torque my head studs to 150 inch pounds, approx 12 foot pounds. (twelve inch pounds in 1 foot pound)
More will chime in, those will keep you busy for awhile.