We had Easy Rider on free-to-air TV over the weekend here and there's been a fair bit of discussion over it on other bike forums.
At the start the guys rock up on their traillies:
I reckon Hopper's bike might just be a Norton. Two pipes up the side, big transfer case. What do you reckon?
(I've enhanced the image as much as I can, but you can't make pixels out of nothing, it was only standard def.)
I think a bit later on, there might be another shot where it's on the back of the ute, but under a tarp.