Ditch the plastic headset if you want to actually ride the bike. A normal headset swaps in just fine.
Thanks - it's for another bike that I am "restoring" - the plastic lock nut has cracked.
I could replace it with a metal set but then I would loose the special ride characteristics of the front end.
Not that I am planning to ride them a lot, I just bought them out of curiosity and something to do in lock down - probably get another as they are cheap and quite interesting to take to pieces, improve and reassemble.
I've been searching the internet for history but there is not much really even though Harry's sons have a "web site" which is hardly of any use concerning the past.
I prefer to replace parts with original spec ones, if they are available.