We carried a Vespa across the SoPac, and other than having to register it in NZ, it was a lot of fun. I drove, with the kid between my legs and the wife behind. Got in some island touring that never would have happened if we'd had to use local transport.
The thing was big and pretty heavy, but it fit nicely down the forward hatch, and though it was work, it could be hoisted out of there and swung onto the dock fairly easily. It didn't hurt that she was a gaff rigger, so we had lots of block and tackles aloft, though not one winch aboard.
I would suspect that these days, if you are sailing internationally, you'll need to get it registered and possibly even pay duty, everywhere you put a motorized vehicle ashore.