It IS an RB80 kit:
So it IS a 66 cc motor-
The 49 "sleeve" Carb you pictured later- I HAVE one of- in fact the carburetion was not good. I just had a thread a few days ago about finally replacing it:
because, yes it was difficult to take on and off the wider long studs that ran clear to the air filter- and they had a spacing wider- AND they had a unique jet you couldn't change.
But I think it is 40 mm and I think you can get a billet now and just put on another carb- I replaced the jug before I knew that. Anyway, now it runs great
But I'm asking about the newer 66 motor shown here