Essentially the workshop/spare parts manual as the modern rider knows it didn't exist in 1921 but it is no great problem as the English Lightweight of the period is about as complicated as a wheelbarrow.
You can get a copy of the 1921 catalogue from the English Vintage Motor Cycle Club plus as Radco used a proprietary make of forks and gearbox you will probably find they have material on those items as well.
Radco used their own make of twostroke with little if any alteration for years and years so any literature on the engine will do.
Basically, if you are in England, join the Club and you will meet other owners and riders of similar types of lightweights all of whom will be pleased to meet you , assist you and initiate you into the pleasantries of proper vintage motorcycling.
There are many things to learn that aren't specicfically Radco. Belt drive care, how to deal with beaded edge tyres and so forth. It's a steep learning curve but so much fun.
Cheers, Bob
