Chris has some cool ideas but that displacement kit is going to cost you. Make sure your moped is worth the investment.
Here's what I've done to the Minarelli V1 on my Cimatti City Bike:
1. Replaced the stock exhaust (which I blew up, anyway) with a Gianelli performance unit. I had to order this from a guy in Italy who has provided very good service to several people in the Moped Army. Chris claims that the Morini unit can be modified for the Minarelli engine, but I don't have any experience with it and I was personally reluctant to try it. That may or may not have been a mistake.
2. Replaced the stock 14.12 carb with a 15.15. You can get a new one from MWH, or others, but I picked up a used one from the same guy in Italy.
3. Replaced the front ten-tooth sprocket with an eleven-tooth, which is the largest that will fit on my bike. Chris is looking for an appropriate rear sprocket for me to replace the 42-tooth unit, since the change in the front wasn't large enough to make a significant difference in top speed.
4. IMPORTANT: If you make either of the first two changes, you are going to have to rejet your carb. The existing jet will almost certainly be too small, which means you'll be running too lean and risk a seizure. I'm still in the process of figuring out which jet is best for mine, but I started with a 68 and then went to a 66, 64, 62, and now a 60. I'm ordering a 58 and 59, one of which will be perfect. BTW, my setup had a 52 in it before I started all this mucking about.