Whatever it was it was illegal. It looks like a bolt on conversion kit available off
eBay. It would have to be subjected to an MOT test and I doubt that it would pass because there is a large expanse of unprotected transmission chain amongst other failings.
There were legal version from the 1950's with the engines perched on the fork, built into the wheel and if not direct drive, transmitted the engine power via rollers onto a thick treaded tyre.
Velosolex were once very popular. I came across three of them ridden in a group by some French pesioners on Eurovelo Six last year. The bikes were from the 1960's