Green’s Dictionary of Slang

nifty fifty n.

also niftyshifty
[nifty adj. (2)]

(Irish/US) a 50 c.c. scooter.

[US](con. 1968) Bunch & Cole Reckoning for Kings (1989) 211: Mosby clambered on the back of the cowboy’s niftyshiftyHonda50 and the Vietnamese gunned away.
[Ire]G. Coughlan Everyday Eng. and Sl. 🌐 Nifty 50 (n): a Honda 50cc motorcycle.