I've found that the only shops that want to work on moped cylinders are ones that work on kids dirtbikes. Find someone that sells 50cc dirtbikes and ask them where you can get an overbore done. Then get that mechcanic to determine how far to overbore it, then get a new piston and have the work done.
The downside is that you are looking at hours of legwork, months of waiting, and easily over $100 is parts and labor. Maybe you should just get a new kit?
How bad is the seize anyway? Cast iron is usually pretty robust, you may not need to over bore it at all.