sometimes it is difficult to accurately measure tubes even using a digital caliper. below is a pic of a brompton that has a chris king gripnut headset. the dims for this headset are listed as:

[TABLE="class: specTable, width: 450"]
[TR="bgcolor: #CCe5ff"]
[TD="class: header, width: 80, bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF"] [/TD]
[TD="bgcolor: #FFFFFF"]total
stack

height

(mm)
[/TD]
[TD="bgcolor: #FFFFFF"]lower
stack

height

(mm)
[/TD]
[TD="bgcolor: #FFFFFF"] [/TD]
[TD="bgcolor: #FFFFFF"]head
tube

I.D.

(mm)
[/TD]
[TD="bgcolor: #FFFFFF"]base
plate

I.D.

(mm)
[/TD]
[TD="bgcolor: #FFFFFF"]fork
crown

O.D.

(mm)
[/TD]
[/TR]
[TR="bgcolor: #CCCC99"]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[/TR]
[TR="bgcolor: #CCCC66"]
[TD="bgcolor: #Cce5ff, align: right"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[/TR]
[TR="bgcolor: #CCCC99"]
[TD="bgcolor: #99CCFF, align: right"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[TD="bgcolor: #CCCCCC, align: center"][/TD]
[/TR]
[TR="bgcolor: #CCCC66"]
[TD="bgcolor: #Cce5ff, align: right"]1-1/8"[/TD]
[TD="bgcolor: #FFFFFF, align: center"]109[/TD]
[TD="bgcolor: #FFFFFF, align: center"]33.9-38.9[/TD]
[TD="bgcolor: #FFFFFF, align: center"]13.7[/TD]
[TD="bgcolor: #FFFFFF, align: center"]34.0[/TD]
[TD="bgcolor: #FFFFFF, align: center"]33.9[/TD]
[TD="bgcolor: #FFFFFF, align: center"]30.0[/TD]
[TD="bgcolor: #FFFFFF, align: center"]30.1[/TD]
[/TR]
[/TABLE]
[TABLE="class: specTable, width: 450"]
[TR="bgcolor: #CCCC66"]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[TD="bgcolor: #FFFFFF, align: center"][/TD]
[/TR]
[/TABLE]

you can use this data as a guide. it's possible that the 90's era brompton had some different dims but, most likely any conventional threaded headset with dims similar to the king headset will work for your bike.


here is a generic headset that's listed as a "scooter" headset. although it is probably just a conventional standard 1 1/8" threaded headset. you can contact the seller for dims. coincidentally, they are based in australia.
if it works, it would be cool as they are sold in a variety of anodized colors..