Re: Gas or ebike?
I looked into electric originally myself, I have even built a small solar set up that I could use to charge.
When I reviewed all my options I decided the best bang for the buck for my needs was a four stroke kit. There are a few a few vendors on this sit that sell kits. If you go that route make sure you buy a kit with a decent gearbox...Grubee 4 or even better the Q-matic, just depends on your budget.
If your you want to push toward 40mph you could also look into a shift kit, SBP seems to be the named favorite on site. Do a few mods, change a few sprockets, and you can get to what you are looking for speed wise...just remember you would be doing 40mph on a bike.