Some BF folder members think old R20s are a waste of time and money and prefer modern bikes, however, the great Sheldon Brown was a keen advocate of the venerable R20 and modded quite a few, so......
Are you keeping it standard or modding it? If you're modding then Sheldon's info is the best to go by, a few of us R20 owners have used his knowledge to guide us through some of the problems with the R20.
I'm not sure how much my R20s weigh, but they are heavy, which isn't necessarily are bad thing....built to last?? I've toured on mine with loaded panniers back and front and I've appreciated the solid build, not the flexy feeling you might get from a modern folder. If you gear them up right and you're a reasonably strong rider the R20 is a good solid workhorse that will last and last.
The 76mm wide BB shell is threaded 26tpi, not 24tpi like most bikes so it does present some problems if you want to use a modern cotterless crankset. There are a few ways around this:
1: Buy a Phil Wood 73mm BB with a 113mm length spindle and Phil 26tpi retaining rings.....very expensive!!
2: Use the Phil 26tpi rings and find a 73mm Shimano UN72 BB (113mm spindle). The UN72 has a threaded cup which is removeable, the other end might come off by a few gentle taps with a rubber mallet, or if not, very carefully using a hacksaw cut it off along the length. This will make the BB ready to use with the Phil rings.
This method is cheaper, but the availability of 73mm UN72 BBs with the correct spindle length is becoming increasing difficult.
3: Find a LBS you trust who will cut the BB shell from 76mm to either 73mm or even better to 68mm, (the standard BB shell width), get them to rethread the shell to 24tpi and you can use any Shimano BB you want.
This method is probably the best way to go, many R20 owners are choosing it and not having any problems.
Next is the rims and brakes.......
R20s in Australia and I think in the UK came with 451mm rims, not the BMX 406mm size.
If you're changing to narrower 451mm alloy rims then the standard R20 brakes are not the best to use, so a modern brake system is advisable. This presents some problems if you're using mudguards as road calipers will be too short and narrow to reach over them. Tektro make a long reach dual pivot brake that should work, Shimano might have some as well.
Not using mudguards?......READ Sheldon's article on "drop bolts".
For my modded R20 I use Shimano 105 calipers with Sheldon's drop bolt idea for the rear brake, and for the front I inserted a length of steel tubing up into the steerer tube, drilled a hole so I could bolt it into the existing brake hole, then drilled another hole to fit the new caliper at the correct length....works perfectly.
The range of tyre and rim choices is limited in the 451 size, so you might choose to go 406mm.......
406mm
Better range of rims and tyres that are more common worldwide, any LBS will have them at all times.
Use Sheldon's drop bolt method for the brakes, or as some R20 owners have done, me included, braze on Vbrake mounts for an excellent braking system.
The next thing to change is............
Just remember the R20 is an old bike...you can sink some money into them and mod them like I have, it will still be an old bike, but if you're a tinkerer and you enjoy some challenges the end results are a surprisingly fun bike to ride which will serve you faithfully for years to come.....and NO FLEX!!
Good luck.