I think it was either Gilera or Aprilia that campaigned a 125 twin in some GPs around the Gaston Rahier / Akira Watanabe era of GPs.
Twins were rapidly banned, as bringing a potentially damaging level of costs to MX racing...!
Yamaha tested with a twin 125, and of course Honda had their twins in both 125 and 250 capacities. I think they used them for a time in the Japanese nationals. Twins were banned until Aprilia came out with their twin 450s / 550s. It's amazing how quickly the rules can be changed, especially when the main factories don't see the new bike as being a threat. I think now there is a 'gentleman's agreement' between the big factories to not pursue twins. Mind you, I think you'll see KTM and perhaps at least one Japanese factory come up with a 450 twin for Rally / Raid racing, with how well the Aprilia did in the SA Dakar. Probably the most exotic MXers ever made were campaigned the the J Nats - and you still see the trickest bikes there. Honda, for quite a few years experimented with CV transmissions on some works 250s - the nature of which was shown , ironically in Honda's patent application for their internal gearbox DH bikes that Greg Minnaar used to win the DH World Cups.
It was a variable dog bone /scissor link set up, that I've got a patent listing copy somewhere in my workshop - it was about 100+ pages of very interesting stuff. The funny thing is, the works Honda DH bike gearbox ended up , in its last few incarnations, as little more than a internal derailleur / sprocket cluster set up. It was hard to top something as simple as a system just derailing a chain onto a variety of sprockets. But there are few things that can stress/ prove/disprove the action/ feel and power transmission/efficiency of a drive line than the (in relative terms) ultra high torque and low revs of 2 human legs.
