Diesel or gas engine as an e-motorcycle range extender?

I read with interest the old thread “motorcycle engine as an ICE range extender?”

I’ve got a scaled-down version of the question.

I’ve got a 2011 Zero Motorcycles Zero S electric motorcycle that just can’t make the 38 miles once-a-week trip I bought it for. It can do about 30 miles.

So I’m thinking I could put a 5-7HP engine on it, geared so I can mechanically engage it and it’ll hit optimal RPM at 50-60MPH, just for the freeway portion of the trip. Optimal RPM appears to be 3000-4000RPM for these 30-40lb engines)

I’d rig it so any braking at all disengages it (brake light wire is handy for that).

Any advice?

I have looked at generators in the same class, and they weigh about double.