Look on ebay u can find some real cheap....
I wouldn't go much bigger than 30's with the mx setup...
pwk "race carb" keihin....
Head depend on ur elevation.....any aftermarket head is good....
"Noss" is real good ...wink,wink,wink!!!
If u r into Mx, I would look into A...2 in 1 carb setup for more bottom end...
graydon or trinity make a good 1...