Was digging and found this, performance parts and a 30hp GX390 lol!
http://www.parsonsracingengines.com/page/engines
Also found this site that has performance parts for it too
http://www.affordablegokarts.com/mm5...ory_Code=GBPK9
I emailed one about a high rev kit for carts, they are running like 6,000 rpm no problem but not sure how much stuff they changed. 5K rpm in a cart would give great speed. Also one place says just machine the key down to advance timing.
One also has a cheap spacer ($10 iirc) to get a signal from the intake to run the fuel pump. There is another one that is large like one inch thick, they say it adds power at all rpm because it makes the intake longer.