Yes, no, yes.
Yes. British Leyland only adapted one engine for North American emissions standards. They chose the 1500 for both cars.
No. It's the only engine similar. The most common Midget engine is the 1275 A-series.
Yes, 1500's came with a single Zenith Stromberg carb. Nasty bit of work that. In Britain they came with twin SU's
BTW the transmission is different from a Spit.
Not sure why you're curious but you'll get more answers on the Spridget board.