Hi Merv,
I'm in deepest Suffolk! Not sure that the RD250E (might be an F as its genuine yellow?) is viable project with a 400 motor in it? I'm guessing the purist's would say that its a wrong-un! I believe the main differences between the 250 and 400 are rubber engine mounts, exhausts, carbs, airbox and perhaps the swingarm length - is that it?
I'll get on the aircooledclub site as well.
Cheers.