from near Salisbury, UK.

Are there any numbers stamped into the carb body? They'll be on a flat surface. These might help identify what that carb was originally fitted to.

That carb is similar to a Dellorto, commonly fitted to the likes of kids Italjets, Fantics, Garrelis ect. The type where there is no airbox, just a mesh and wire wool screen over the carb mouth.

The carb fitted to the TY80 was a clamp on mikuni VM18 and I believe the GT engine is essentially the same as the TY, barring gearing and stator. Certainly looks like it.  Seem to remember a plastic sleeve fitted inside the spigot clamp..  Long time ago.

No idea of jetting differences, but that is not hard to research.

Ok, one more try. That chinese carb i thought said 15mm, which it does, but when i take screen shot & expand it up the 15mm refers to the venturi size down the end where it steps down at the end of the insulator spacer. Therefore i'd take a guess that the adaptor, insulator bit would be 20mm that you require.

Aliexpess says this carburetor is for Yamaha PW80, which has 20mm intake side, so it supposed to fit. I will order it, and we'll see how it's gonna go.