I have a 2005 GruBee 48cc round-head "Wing Ding II (also called Pico 50) engine only on eBay right now. This is from an unused kit that has been sitting on the shelf for four years. We're selling the engine from the kit because we're using the other parts from the kit for a four-stroke project.
The engine comes with the carburetor, mount extenders and extra spark plug. If you have a China Girl that's on its last legs, here's a great replacement engine. I have one of these on my '80 Western Flyer, and it's a strong runner. Here's the link:
2005 GRUBEE 48CC WING DING II / PICO 50 BICYCLE ENGINE - eBay (item 160361192876 end time Sep-14-09 19:11:48 PDT)