Actually I thought about the CR barrel conversion, I have 2 CR125 barrels at home, The conversion is not as straightforward as it sounds (even though the stud spacing is the same) and the YZ250 conversion give a lot more CCs. If some kind of efficiency can be achieved, it could make big power, for a bit.... or go bang... or vibrate itself to death.
There are a few people doing CR125 conversions on the NS400, but I haven't seen one running yet or seen a dyno curve. If someone builds one and it's the dogs, I'll reconsider.
I rang Dan about the one he's selling on ebay, he said it's cost him about £2500 just on the engine on his 500cc CR/NS400. For a similar outlay I want to get a 650/700cc YZ engine built if possible.
Cheers, Mat.