RE: Grpro 100cc twin. or?
Is it Rear Induction or Side induction?
If possible - take off one cylinder and look to see if it has rollers or bushes on the connecting rod.
Due to significant improvements being made to their product lines, The RCGF factory recently sold off their entire stock of early model engines to a 3rd party "generic brand" dealer (you might be surprised at how often that happens amongst some of the manufacturers). It is possible that they have fallen into the hands of others who are now selling them as their own brand.
All Current Model RCGF Engines have the RCGF Trademark and a Serial Number. That was only instigated about a month ago.