I’ve got three options for the final drive:
1. Concentric belt drive through the swingarm pivot.
2. Chain drive from motor shaft (in same place as existing pinion).
3. Motor onto gearbox input shaft (in same place as existing clutch).
The first one is the most desirable, but would mean fabricating a new swingarm and changing the swingarm bearing arrangement on the frame. The second one is relatively simple, requiring a maximum shaft speed of ~4,000 RPM. The third one could be interesting: if I used first gear as a reduction box, it would allow use of a much smaller motor (at the expense of having to retain the crankcase for the gearbox).
Second option is probably best for now…