Lovely bike and as Cardan says, capable of far more than 15-20mph.
A useful place to start for information is the VMCC library. They have a 1921 Villiers Spare Parts list which goes through the successive Mks I,II & II 269 Villiers engines. There is a lovely little hand drawn illustration of each part. It will help a lot with your engine. Remember, this is very early days of motorcycling and workshop manuals, etc, barely existed so it's all pretty basic.. It was fairly common practice for the motorcycle manufacturer back then to refer owners directly to the manufacturers of each item rather than put out their own owners manual so expect to go back to the VMCC Library for info on the carb, the magneto, the forks, and so on.
The VMCC also has a Royal Ruby marque specialist who should be able to help. Of course he may be keen for you to become a member but that's between you and him.
The VMCC Library also has complete sets of the magazines of the day, The Motor Cycle and Motorcycling. Well worth a day trip and browsing through the relevant years.
The other club of note is the Sunbeam Club, they also are vintage and veteran oriented and also have a helpful library but not to the depth you will find in the VMCC library. Still a very good Club and just as helpful. They also conduct the Pioneer Run for veterans and do dating certificates.
Final thing, the VMCC shop is a prime source for early belt driver specialty items such as brake blocks, belt rims, belts, be tyres, etc.
Sounds like an advert for both Clubs doesn't it. I suppose it is as for maximum enjoyment of your machine you will end up in one or t'other if not both.
Cheers,