problems with my 250 shifter kart

[250mark]

I had reputabe engine builder for gp bikes modify my 2002 cr250 for my shifter. After numerous attemps of getting it to run over 100 mph on a straight track I gave up an sold it. I then bought a 2002 yz250 and had it modified by FTZ Racing. Bigger bore. port and polish intake and exhaust, modified head, upgraded piston, pvl ignition, custom exhaust, upgraded carb. After breaking it in, I was still diappointed in the performance. I noticed a lot of single cylinder users use the cr250. I there any reputable engine builders in the U.S. that could build me a decent running engine. Thanks!