The frame...

 

That doesn't make sense. If what makes the bike a Harley Davidson is the origination of the engine, then that engine would need to originate from Harley Davidson.

S&S engineers their own motors as direct replacement for OEM applications and custom builders. Rev-Tech, Ultima, and a few others do the same. Their motors work just as well in HD frames as they do in Paughco or even some metric frames. Putting those engines in the other frames doesn't make them Harleys, does it?

Maybe I misunderstood, but your stance would mean even if I stuck a Honda 45deg v-twin into the frame, it would still be a HD. Something about that doesn't seem right.

 

It's a Harley when the VIN starts with 1HD.