Bromptons suffer from flaring in the head tube with the lower cup working loose eventually. IIRC Rogerzilla once suggested a way to tighten the head tube if it's enlarged using a dot punch.
HTH
I used a über-strong grade of Loctite in the end. If you want some I have an unopened tube. A Coke can shim doesn't work (it is great for ISO frame - Raleigh or JIS cup conversions, though) because a stretched head tube is no longer round - it ovalises F/R.
If the head tube is well faced, a Tange Falcon cartridge bearing headset would be a better replacement. It's light and immune to "indexing". Very low stack height of 34mm, and you can grab a little more room by buying a 0.5mm spacing washer to replace the rather thick stock one. I've just put one on a '92 Kona.