Apart from the scooter frame itself and a few lengths of strip metal, that was all bits of lawnmower...
I see people using lots of types of engines for minibikes and scooters. I have a few engines around here, an old weedeater (not sure how well that'd do but it'd probably work quite well for an assistive engine for a bicycle) and a chainsaw motor, which would be a bit more promising but I'm not sure I could tolerate the two-stroke wail.
The scooter pictured there had no muffler, simply a large expansion tube (1-into-2) with some steel wool stuffed into one of the pipes when it needed to run more quietly.
With the choke pulled part out to idle lumpily it sounded like a miniature Harley, hence "Hardly Davidson"...
I'm sure something more tuned would have worked better but it was economical
and fun, which to me is a win-win situation!
I think engine assist bicycle is going to be next up. Living in the flatlands does have some benefits, 3cc's with 2hp can get you moving and keep you moving at a steady pace
--Phil