I have a few Raleigh Twenty bikes. Though there all the non folding bike.
I like modding bikes. Especially bikes like 20s that have some unusual parts that are needin changed anyway. Or limit the performance of the bike.
For instance the UK market 20s had 20 X 1 3/8" wheels. These are ok. But they were steel rims on a 3 speed hub. With a steel front hub thats narrower than modern hubs, and has a smaller diameter axle, 5/16" British standard cycle thread. Instead 3/8" axles that are more common these days. So this means that you need a different spanner to take off the front wheel. Well you can get larger wheel nuts to fit on the 5/16" axle, that takes a 15mm spanner.
The bottom brackets a steel tube, Not too thick metal. Its over 70mm wide. It has a different thread pitch than usual. Its possible to get axles that fit into a 73mm wide shell. But getting them to be secure in there, either means to rettap the shell. Which is a bit risky as you may damage the frame and the shell isnt as strong as other bikes are.
You can use the original cups on a different axle. Ive done this a few times. Its ussually ok. The Raleigh cups seem to be very strong as Ive not seen any that were worn.
Or you can get Phil woods cups to use a cartridge Bottom Bracket. These cups are expensive though. And most cartridges these days have one cup formed on the axle. So isnt removable.
I have narrowed and retapped a few Raleighs to the more usual 68MM width. But its fiddly and now Im not working at the bike shop i dont have access to the BB tap.
Also if you replace the 451 wheels with 406 wheels, they are a little smaller. So the brakes need adjusting, or replaced. I have modded a frame to use 406 brakes by replacing the flat metal brake bridge with a tube bridge nearer the wheel. The shorter brake arms are, the less flex there is in them. Raleigh used single pivot brakes on the 20. Commonly pressed steel long reach sidepulls on the UK bikes. Although Ive seen Alloy brakes on some of them.