Run a F7 motor out of a Cat mated to a LT trans. You may add 4" in with, but it's a very low CG and a very good single pipe setup and you could kick start it.

Then add a fluid dampener to keep your arms from going numb. The Ultra Lite guys started using them in Rotax. We hand made one for the last 1500cc triple we made. Cat 900 twin (1010 big bore) +1 cyl.