lot of people ran Mikuni carbs, so no big deal, and your frame is definately aftermarket,
many guys would hardtail their stock frames, but the frame you have is an aftermarket, the motormount is different, the tube running from the neck to the motormount would not exist on a stock frame, etc.
once you have it blasted, look it over carefully, you may find a number somewhere on it
the ironhead frame was, for the most part, the same from 1957 thru 1978. few minor changes, but nothing drastic at all. I dont know Canada law, but in the US, any motor after 1970, put into a custom frame needs to be retitled as a custom build. otherwise my 74 motor would be in an aftermarket frame