Just finished putting a Polini 64cc Kit with Technigas Next exhaust and Bing 15mm Carb on an old Penny's Pinto. (Thanks Steve's Moped and Bicycle World and Treats HQ on Ebay)
Here are a few observations, though this is probably old hat to a lot of people...
1. I didn't radius the crankcase, so I am missing out on a lot of induction volume, but it didn't seem too difficult to do with a Dremel tool or similar.
2. The instructions for the Polini kit are not great. Just FYI, the rectangular port in the piston is the top (12:00 position for the piston) with the kit.
3. The piston is a single ring piston and the ring gap goes at 12:00 too. The pin around which the gap goes in the piston is very subtle, but its there. So be aware that the ring gap must be straight up when assembling the engine.
5. The 15mm Bing requires the little "spigot" on top where the cable threads in and the banjo fuel line fitting from the original to work. Its a good idea to have new washers for the banjo fitting on hand if you intend to go with a Bing carb.
6. The 15mm Bing comes with an 82 Jet which works ok with the Technigas and Polini (with stock airbox) but does have a little flat spot off idle. The Polini instructions recommend a 70 size jet. But from experience, this is too small. The solution is somewhere in between.
7. The Technigas-Next exhaust will not allow you to pedal the moped. There is a clearance issue between the pipe and pedal arm.
I am sure I am missing stuff, but feel free to add!