Well it's nice you seem to understand why I find these Great Depression Midgets more interesting than autocycles. They are motorcycles, even if rather feeble ones, not powered bicycles.
Several factories made them, Coventry Eagle, Dot, Excelsior, Gloria, (Triumph pseudonym), Sun and Wolf and all got through the depression years if not the war that followed. Survival rate is better than you think and the spares situation is better than most. About the only engine parts not available off the shelf are the plain bronze bush main bearings. The Albion 2 speed box is much the same with the only ball race in the box being a common metric size number 6204.
I believe the trick is restore them realistically. To the standard they came from the factory. These were the cheapest motorcycles the trade ever built. To my mind that carandclassic Excelsior is wildly over restored. I find that level of restoration misses the point. Charterhouse Auctions had a far closer to original interpretation of the same Model 0 Excelsior for sale last year. Lot 238 in their August sale last year. I don't know if you can still access it on line.
Mostly these Midget powered bikes were offered in the 1931-35 period. Villiers introduced their 123cc 8/9D 3 speed unit construction engine in 1936 at much the same price as the Midget/ Albion gearbox combo and more or less swept the Midget powered bike away overnight. Midget engines still sold well in other areas. For example, most that turn up today are ex-lawnmower.
Researching these little beastie was part of my Covid 19 lockdown entertainment. I was surprised at how much I turned up and how many survivors there are. They have survived just as well as any other bike of their times.