To run aftermarket rear wheels on a 50cc Maddog you have a couple of options.
1st option is to get the Monstergy6 12×8 hub kit and a 12×8 wheel. This is a very good kit And runs the rear true centered.
Another option so you can run a 12×7 3+4 wheel is buy a Kymco Hub
The kymco Hub is 4×140 so you will need to buy a 4×137 wheel and take a few strokes of a file to widen the hole to fit the 4×140 hub. The wheel will need to be 12×7 3+4 offset 4×137
This is an awesome wheel that you can run with this hub. You will need to run it flipped backwards.
Another option is to run an ITP Delta Wheel in 4+3 offset flipped backwards. On the stock hub. This will run about a 1/2 inch offset to the right.