Fascinating little car. They used the 3 cyl 492cc Excelsior 2-stroke that was developed at Charles Pinter's (of Berkeley "fame") behest which was in turn developed from the 250cc Excelsior motorcycle engine.
The 492 was created by adding another cylinder to the 328cc variant of the Excelsior as used in most Berkeleys. Excelsior patented the 3 cyl as a "modular engine", and with its built up crankshaft theoretically could have expanded it into a 4 or 5 or "n" cylinder engine. (How's that for arcane, useless, knowledge?)
It's a great little microcar - very rare for sure.