Hi,
The last week-end I saw an unstamped diecast engine. The engine number place was completely blank and slightly grainy, like Honda original.
This prove, at least in France, that Honda was able to deliver "blank crankcase" probably for replacement.
At the contrary, a few weeks ago, I saw a K2 with a false number stamped, a figure was missing !!!
This prove that it's possible to restamp a crankcase... It's true that the character font did not seem to come from Honda
Concerning frames I think that it's easier to erase a frame number when restoring... At least in France.