Whats a good way to attach it then? Would a belt system with a torque converter work?
There are several methods in use.
The most simple approach is to put a roller of about 1-1/2 to 2" diameter on the output shaft of the motor and mount it so that presses on your tire. A lever gizmo pulls the roller off the tire when you want to stop or ride by pedalling. You can also incorporate a centrifugal clutch. By selecting a roller of the right diameter, you can optimize the "gear ratio" of your drive -- smaller roller = slower speed but more hill climbing ability. The old Solex mopeds had this setup. Also several add-on bike kits use this method, either on the front wheel or the back wheel. Disadvantage: tires wear rapidly. A design that is too simple will put too much force on the motor's crankshaft bearing and cause premature failure.
Add a centrifugal clutch and a small pulley on the engine (may be available as a single unit) and drive a large pully on the wheel via v-belt or cog belt.
Add a centrifugal clutch and reduction gear with sprocket output, and drive a sprocket on the wheel. A few folks have figured out how to mount such a setup to drive the rear derailleur, for a transmission effect, and terrifying top speed.
Some European mopeds use a "variator", which is a belt-driven CVT transmission, like a snowmobile has. They work well. Other European mopeds use a 2-speed automatic gear transmission, which also works well. Some are single speed, which makes them slower.