I think the Briggs Animal is the best bang for the buck! About $500 if you shop...just a bit shy of 10HP at 6200RPM, add oil, header, clutch, and let it eat! I have one with a TAV2 on a Murray track2 which is a heavy bike...and I am a big kid at 280lbs....it will lift the front wheel at 15MPH, make me squeal like a little girl, and do it consistently with just a pull of the starter handle. I can leave it sit for a month, or two,....give it a couple quick pulls and it runs like a champ!
just my .02