When I was young - late teens/early 20's - there was a group that tried to get a series started in Ontario and Quebec. I don't think we ever had more that 6 outfits out so it didn't really catch on although its huge in Europe.
My dad was always into sidecars and raced with the SRA for a number of years but loved MX. He had a Rhind-Tutt WASP frame with a 920 Norton that ran on methanol. It was very old school by today's standards. It weighed a ton and probably only produced around 50 HP. I remember it had the remote oil filter bolted down to the floor and PiperCross air filters and pipes (are they still in business?).
I'm going to see if I can get my brother to dig through the family photo albums and scan a few pictures.
Now its all purpose built single cylinder motors. For a while it was almost all 700cc Zabel monster 2 strokes. Now it seems that the TM 4 stroke motor is the thing to have. I still think it would be quite fun with the TM 450 as a starting point. They do still race them in the US and have been to see them at Glen Helen a few times, but its still very niche in North America