This is probably outside what you are looking for, but I like to give options. Sounds like you haven't started the build yet so here is an alternative idea.
You can successfully modify a cranbrook. that being said, you should compare the pro's and con's of of just buying a bike that has what you need.
For example the
Schwinn Lankmark, and the
Schwinn Clairmont are both excellent bikes to motorize. I have perosnally done both.
pro's
Multi speed (I like it, you can peddle at top speed for the extra rush)
Has front and rear brakes
Don't have to mod the coaster
Don't have to buy seperate brakes
Have to buy dual brake lever either way
Con's
Little more expensive. $149 for the landmark, $159 for the clairmont.
Not the brake style you want
Probably have to buy the ones I listed online. My store never stocks them it seems.
I'm sure there are others, but those are the main ones. At $90 for the Cranbrook, how much are the brakes gonna cost you, and how much headache are you willing to get, modding the coaster. Some have better luck than others with the coaster break. If you destroy it, how much is a replacement?
The Cranbrook has a place for being motorized, but there are better alternatives for a little more.