FWIW, I bought a Citizen 16" folding bike for $200 including shipping and a carrying bag on the web.
Then, I hung my BikeMotorParts.com friction drive ($175 including shipping) with a Subaru/Robin 33cc 4 cycle engine ($100 on Craigslist) on it. Top speed is about 28 mph.
It all fits easily in my homebuilt experimental airplane.
Now, when I fly somewhere, I have 'wheels'.
(When it's not on the folding bike, the friction drive is on my Rans recumbent.)
Very cool motorized folding bike - under $500.