Here's the two Franco Morini S6 engines I meant to post a while back. One is 90% complete, the other would be good for parts or a rebuild. Both are the tame versions of this engine, but one can simply bolt on the reed inducted top ends (3rd transfer needed for SC version). Neither engine has a clutch, but those are simple enough to come by...
Single speed auto/ 49cc / HP in the teens with reed top end.
They're tiny too.
Magnum tank for size reference only : P
$180 shipped obo