XR250/400 have the same clocks so ignition mounting is likely to be the same.
I'll search a box of bits tomorrow, but I don't remember seeing the part for at least a decade so don't hold your breath. I'm sure it is there somewhere, buried quite deep.
Maybe not Russ but you will still have that bit of 2 stroke fun. Ive seen Dave out on it a few times and it looks a tidy bike and it certainly goes well.
After the CRM was sold I went up to Brighton and bought a 300 EXC, now that is fun!!!