My Mongoose Mack was a tight fit. I had to shape the front motor mount to fit the frame and shorten the intake about 1/2" but I got it in there. Where there's a will, there's a way.
I plan on doing a Schwinn Kokomo for my next project. If that thing in the upper frame is metal that would be perfect. I will fab up 4 right angle brackets and the gas tank will be really solid.
The Mack has disc brakes front and rear so I went with a jack shaft but the Kokomo will probably be direct drive, I just have to see what is there to mount a sprocket to. I have a sample Zeda Firestorm on its way from China to power the Kokomo.