That is mighty cool of you to have done that.
That kinda "if not me, who? If not now, when?" sorta thinking and doing is in short supply IMHO. I really do believe that a whole lotta bad in the world could be fixed with a very little bit of good. Just like that.
I kinda think you answered your own question and should motorize a bike you specifically would like as a MB. At first it will seem like a great expence but if it will bring ya more joy, big time worth it. I tried doing the math once. But based on 2 smoker kit price and living 6,000 miles on a $150 bike, worked out to some thing like .5 cents a mile for months and months of fun. Dunno, just saying.
Lots of great used bikes for sale too. Clist, tag sales. Can find some awesome ones and/or parts to build one at the dump.
Post lots of pics!