Recently saw this new UK company mentioned on Autoblog. Pretty cool, but a bit spendy, and of course not in the US (yet?). They use a 500cc thumper from the Buell Blast, which could work well with the presumed light weight of the bike. Can't find mention of actual specs for it. These could be an inspiration for someone building a retro cafe bike.
Autoblog article about Mac Motorcycles, w/photo gallery
Mac Motorcycles
Autoblog article about Mac Motorcycles, w/photo gallery
Mac Motorcycles