400cc? Wow! I figured my GG321 was big. I can't imagine what sort of flywheel he had to put on it to slow it down enough to really ride it, then what would he do with that power? I wonder what it weighed? I've shaved off every bit of weight on my TY that I could and have it down to to just under 200 pounds. It actually works pretty good. The motor is a little slower than today's bikes, but other than that it's pretty competitive. It was fun building it. I'm still looking for that Hodaka though. I still have the original sales brochere for a 1970 SuperRat.
I wonder which would make the better Trials bike, a 100cc or the 125cc? Isn't the pic you posted of HR's a 125? Cylinder looks like it is.