That's a Tecumseh Star engine with an aftermarket Clements Head and other upgrades. They were roughly based on the 10-HP Tecumseh HM100, but the Star engines were specifically intended for kart racing. They were 357cc. and had around 20HP in stock form, but yours looks to be highly modified by Clements Racing Products. Mike Clements Helped develop these engines back when Tecumseh wanted to get into kart racing, then Mike Clements Racing Products developed and sold all kinds of aftermarket componets for the Star engines, and they also sold complete race prepped engines, and I think you've got one of them...
You'll find lots of Star info doing a search at the Bobs Karting forum, here's link to a cool one:
I run at a fairly long track, with lots of bank to it, most of our opens are gx390 or briggs 342's, does a star motor still have a chance against these guys?
4cycle.com
I used to own 2 Star engines back when I raced karts.
You'll find some more Star engine info, pics and videos on my old racekart thread here on this forum at this link:
I scored this Invader offset/oval racer a few weeks ago but finally shot some pics today. All I've realy done so far is instal my engine. Next week I'll probably be taking the kart/seat over to my much more experienced buddies house to have it all setup and scaled to race at this track...
www.diygokarts.com
Here's a pic of the Star engine on my old dirt-oval racekart:
