Yes, replacement frames could be purchased from Honda back in the day.
I have a 750K3 with a replacement frame.
It has no vin plate and the state-assigned number was hand stamped on the neck.
Fortunately that number matches the title.
The PO told me the bike had been backed into by a road grader back in the 70's.
True story or not for some reason the frame was replaced along with some other parts years ago.
The upside is the HM341 pipes are in almost perfect condition.
I do not know for sure but I suspect the reason my bike has a state-assigned number is that is was likely totaled out by the insurance company after its accident and the PO bought it back and had it repaired and re-titled.
The repair must have been done by qualified shop because I haven't found a single nut, bolt, cable, or wire out of place.
The only evidence of previous damage is the new frame, a slight dent in the oil tank, and repainted side covers and tank.
(And of course the minty pipes.)