I'm looking for motor and gear box combos. I've got the itch to leave this rat race and actually do what I've always wanted, build modern custom cafe racers... So, i'm just starting to look around a bit and get some ideas..
In addition to Derbi, the Honda NSR 50, Yamaha YSR 50, and Aprilia RS 50 are all popular and street legal bikes. Don't know about the others, but YSR 50 engines are on Ebay all the time. YSR 50s are raced all around the country, so there are a lot of them and a big aftermarket even though they have been out of production since '92.
I have a street legal YSR 50. They are too cool. Quite a few folks customize them and stuff in bigger engines of all types and sizes. I don't know if that is the type custom cafe you're talking about. In any case, Google search for ysr50 and several sites will turn up with galleries to look at for ideas.