I'll weigh in again with agreement to what the PARacer said...
although many motorized bicycle riders use the sole coaster brake their beach cruiser was equipped with, IMO, it is quite a bit less than safe.
Coaster brakes can fade, or lock up. They are also totally reliant upon the chain.
The experience upon a motorized bicycle is similar to going downhill, on a pedal bicycle, all the time. All well and good, great fun, yet we have NO control over other people sharing the road with us, especially in traffic.
The best braking system we can get is far more important to us, than them!
If I DID run a coaster brake on the rear I would use a disk brake on the front.
Also, most beach cruisers don't have front suspension, because they are meant for slow, boardwalk pedaling...
As your speed increases, the value of at least front suspension becomes obvious. As with a plush seat! lol
Fun stuff!
rc
BTW, what Dan did NOT say...

... is that 4 cycle motors are much more reliable than the 2 stroke kit motors.
.