If it is based on the Tigress, it's two engines..maybe back to back....Original Scooter Design was a parallel vertical twin, exhaust pipes exited to the side, then carried backwards to a silencer above the rear wheel. Engine was 250cc four stroke. Smaller single cylinder version was 2 stroke. I seem to remember they had an extra foot pedal to select neutral....
As noted, there are some clever folks out there. Chaincase looks an amalgam of Front Plunger, rear S/A. Maybe wet sump conversion, or a central oil tank. Genuine BSA Dripper heritage maintained.
Swarfy.