many on this forum say to stay away from the Alum. Frames for motorizing, I have not used one myself but knowing how easy thin Alum. can fractor under intense vibration I would not trust the Alum. frame nearly as much as I would the steel, but again there are probably several people that have used Alum. frame with good success but I would imagine that the frames they used are thicker and higher quality than the cheap Big Box Store Schwinn type frames, the steel frames of the cranbrook type bikes have even been known to crack with some who have used them, I have one that is a cheaper Huffy frame just like the Cranbrook but so far it is holding up just fine but only time will tell, I say find you a good multispeed bike with a steel frame, Schwinn make sthose also and I believe K-Mart carries those and I'll bet they can also be found on amazon.com.
also Check out the Vendors on the forum who carry the Micargi bikes, they seem to be used quite a bit with the motors. I'm not sure who all has them but I know Pirate Cycles does.
I say skip the Alum. frame......just my $.02