The english equivalent to the Solex, was the "Powa-Pak" which took the place of a saddlebag and drove the rear-wheel via a friction-roller.(heavy -duty bike-tyres were available for these! )
Far better, were the ones inside the rear wheel. The Cyclemaster was completely self-contained and the fuel-tank held only a couple of pints.....not a huge problem when it did about 200 MPG
Though one had to take a can to the petrol-station and buy a half-gallon, mix in the oil (2 stroke) then decant into the tank as needed.
Much more up-market was the BSA Winged-Wheel which was somewhat sleeker and had a remote oblong tank where a carrier would normally be fitted. It's major advantage was the tank being big-enough to take the legal minimum dispense at a petrol-station.
There were many others available, early post-war and the "Corgi" folding paratrooper's machine was a fairly common sight (98cc Villiers power? )-non-availability of tyres killed them off.
No MOT and free road-tax on these machines, now. Insurance will be peanuts. Most are conversions of standard bicycles, though Phillips did a heavy-duty one beefy tyres and wheels and a big mattress-saddle with large chrome underslung springs on each back corner. the crossbar kinked down at the back, allowing a lower saddle-position. still the same cruddy rim-brakes though!
great fun on a sunny day, but too energetic round here. (they were about 25cc )