Golden Eagle does a nice belt drive system IMO with modern, reliable engines in 2 & 4 stroke flavors. There's also a chain drive company using same choice of engines.

If I were looking into ICE kits, I'd consider one of these.

http://bikeengines.com/

Staton uses chain drive:

http://www.staton-inc.com/Results1.asp?Category=30

These "eBay kits" are wannabe Whizzer clones. Look cool but probably not gonna last over the long haul.