I've currently got some RC35 MXCDs for Mrs Rouleur's Orange P7. The mono box grown is sized for a 1 1/8" steerer, but I've got a shim to knock it down to the 1" steerer. Shims and steerer still available from Pace direct if you decide to go that route.
There are a few modern 1" threadless style headsets available too. Ritchey Logic and Cane Creek (who used to be Dia Comp, so know a thing or two about Aheadsets!) do decent ones at the £30-40 mark. FSA for a little more, and Chris King for the big bucks.
I bought a cheapo silver Genetic (or something) from SJS, but just 'cos I wanted something to match the nickel frame while I look for something period correct!