Keeping the shaft drive means taking a major efficiency hit and engineering an adapter plate from motor to transmission. Getting rid of the shaft drive means manufacturing a custom swingarm. Either way, that's not cheap or easy.
If I were in the EU (and if I could fit on one), I'd get a 50cc sportbike like Derbi, Rieju, or Aprilia, and convert that. There would be a lot more performance and range available for any given amount of battery and motor.
The used motorcycle is the cheap and easy part of your project. Don't let an inappropriate choice there determine the rest of your options for you.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.