I've got one DR set up with later single shock 38s and emulators. With hindsight I would go for the early 38s and keep the DR front wheel [though you can use sleeves to use the early wheel with later fork]
I run the first RM 1978 ally swing arm, which didn't need much altering, 16.5 Falcons and a ProTec mild cam and the 370 carb. I'd use a bigger carb for motocross. One problem is working out what spring rate for the cam as no one stocks springs and keepers. I think R&D have the springs but no keepers. In all honesty the main gains are unplugging the intake and exhaust, the standard cam is good.
SON is right, it really needs stroking. It was in production for such a short time that it didn't get much attention, but it's a really simple and tough engine, simpler than an XT, and the frame is RM derived. Handles well except at low speed when the front end wanders. It's not that good at really high speeds, moves around a bit compared with a KLX, but is ok on a motocross track, it's intuitively predictable. I use mine for welsh forest rallies where you get everything from wet steep mud and scree to flat out grade roads. I keep the carb/airbox to keep it a submarine. It's not uncommon to get 3 or 4 " of rain in a weekend.
I had problems getting heavy enough fork springs and the 400 carb not working as soon as I opened up the airbox intake. If you've got the WB 36 mikuni carb already I'd love to know the specs. Tom White sent them to me some 30 years ago and I've lost them.
RaceTech do the fork springs
http://www.racetech.com/page/title/FRSP-RT%20Fork%20Spring%20List
though I picked up a heavy duty Maico set.
