Brompton uses a 1 1/8" threaded headset and very few of those are made nowadays.

The hub will be fine, run some oil into the hole where the indicator chain goes.

I'd change to decent brake levers (not V-brake levers), cables and pads. Make sure the length of each cable isn't too different from the originals.

Check the rear triangle, they tend to rust through.

Some folk used to bolt a smaller ring to the outside of the existing chainring (instead of the chainguard) and hand shift when they got to hilly regions.