I originally put my dax on a large tube cruiser with coaster brakes.
Lessons learned-
Brakes
A coaster brake is not enough to stop a 250 lb rider doing 30. I ate up the brake, broke spokes and suffered constantly from wheel deflection, reulting in torn up tires from rubbing on the chain
opinion
Rim brakes are the way to go as the whole spoke issue and deflection issues are gone.
mounting issues
I had a 1-1/4" tube to mount to, so I used a stainless steel mineralac strap (a common support for electrical conduit) and some dense rubber originally used for isolation of transformers.
Great idea, but the motor rotated horribly on the frame resulting in poor alignment and gear box jams of the chain.
opinion
go with a frame that fits the motor out of the box. Easier, better fit solid connection, and no slippage. Mostly this means an older frame, but hey
you can still dress out in any fashion.
Throttle assembly-
the most dangerous part of the entire concept IMO. Does not fit standard handlebars, it hangs off the end giving a false sense of attachment to the bar. This need work!
opinion
I traded it out for a stubby brake lever mounted like a thumb throttle. I feel way better and safer. I know you can also try a twist shifter by taking out the ratchet function and crimping an end for the carb, but whatever you do, get rid of the stocker throttle.
Other tips
constanly monitor torque of all bolts!!!!!!! I believe during break in, you need to wear a shirt that asks following traffic to honk if parts come off.