For a motorized bicycle, it all boils down on having a good, strong, sturdy frame to fit an engine. I don't know if there are any frame builders in this forum but please if you're out there, kindly reply to this thread.
For those who are interested in designing and building their own frame:
Here's the link to the free software 'BikeCad':
Free Bike Frame Design Software
Jump to the software:
The Bicycle Forest :: BikeCAD
Enjoy your frame building...