Well, that depends on what you want to do.
If you have a bit more budget, and don't want to be constantly playing with the motor or clutch, get a gebe or staton rack mount kit with a Robins/Suburu, Honda, or Mitsubishi engine. These motors are workhorses, with great quality (both motors AND kits,) and just keep on rolling along with little 'tinkering' required.
If your budget is lower, or you REALLY like the frame mount look, go with one of the Grubee/Happy Time two strokes (or even one of the 4-strokes) kits. Just be prepared to spend a lot more time tinkering with them to keep them running.
Ref the crash course
Keep in mind that, in AZ, if you get caught going over 20, they can really throw the book at you. (Or, even if the cop even says
you're going over 20!) One of our members, Torques, is looking at jail time
. You see, the AZ law was written so that IF you're going over 20, the motorized bike somehow transforms into a moped - which needs license, registration, and insurance... and which CAN'T ride in the bicycle lane.