I've heard of two; a metro:http://utterpower.com/10hp_chevy.htm
and a festiva with a bit more power:http://www.youtube.com/watch?v=SPuv_6bZKwY
The metro has a air cooled 10 hp chinese diesel (about $500).
The festy has a water cooled 13 hp /22 ft lb 2 cyl Yanmar (currently $750) and weighs around 1600 lbs.
My build will be governed to around 18 hp / 45 ft lbs pulling up to 1000 lbs in a much more aero package. However, if I find the top speed inadequate, I can adjust the govenor for more power. Our Helix can go 75+ mph on 19 hp (7500 rpm) and less torque (peaked at 6000 rpm with 51nm/38 ftlbs). What I'm doing is not that different from Riley's Centurion with a 17hp diesel and spitfire drivetrain, just more power less weight, a more efficient transmission for better accel, less weight and drag, etc.http://www.rqriley.com/cent.html
Instead of piece meal on the transmission, I'm going to buy an atv with a bad engine. The fnr box on the polaris 300/400 2 stroke will work well and has a sprocket output. There is a gear reduction in the box, but I can change the final drive driven pulley to offset the reduction. It appears these boxes would work well for 750cc or less BECs and cost around $200 on Ebay.