its probably a 1" headset (my 400 and 800's of similar age are)
so go for summat similar ,ritchey ,tange ,or so
as for the peps they tend to be a bit stiff in comparison to the cromo
O.E. ones ,if they turn up in the right colour, and price then go for it
sort it love it ride it , get on a retrobike rideout with it
Cheers, is there anything specific I should look for re: headset? Will a modern 1" 'just fit'?
a modern one will be okay in the right type ,but it may just need some new balls and fresh grease