It's certainly the original engine as the Supercut came out with a G40 or G41 engine. He's most certainly one that bends the truth a bit.
There are over 32 variants of the Supercut and had engines from Villiers, BSA and Honda in their line up. In actual fact SB would basically build one to your own requirements, that's how flexible they were with the Model 17.
I find it odd that he sells himself more of an upmarket antique dealer but goes out every Saturday morning and lists everything he finds straight off his front verandah instead of a proper shop front. He also uses that stupid line that "this came from a very wealthy estate", oh yeah ! and that New Yorkish User ID he uses. Some people just get a little carried away with themselves.
Cheers,
BB.