Re: Options to reinforce frame drop outs?
It's also the grade (quality) and source of the metal that makes a difference between a good frame and a not so good frame.
As far as aluminum, I am sure the more expensive mountain bikes use a different grade of aluminum than the aluminum cruiser bikes.
Most of the older steel frames are good for motorized bicycle. Just be aware that some older frames, like Huffy, had pressed fitted and spot welded chainstays. I don't think I'd take a chance without having some brazing or welding done on the chainstays.
Reinforcing the dropouts on aluminum? You could probably find someone to weld, but without heat treatment, you'll probably have more problems down the line.
Go with steel and you should have no problems with having the chainstays reinforced.