Well I've found the answer, but only by pure luck. I wondered if the motor was a Puch of some sort (don't ask me why) and I asked Images for some suggestions, but it came up as "push" engines. I persevered and found an interesting site of old drag racers when in fact I had assumed the puzzle car was a sprint/midget racer.
It's the Hisso Special built by Ron Benham and Paul Aicher fitted with a 4-cylinder Hispano-Suiza aero engine. Would this motor date from WW1?