OK, so I have an engine that runs, result.
Trouble is that my other moped won't play, so Mac is now a test bed.
I've replaced parts a bit at a time and seen what happens.
First was the cylinder and carb from Mabel, no go. So off came Mabel's carb and on went Macs, go!.
It looks like all my problems with Mabel are carb related.
Next, try a high compression head and retime the engine, all's well.
Swap the head for the SP50 and install in Mabel, no go.
Change coil, go. New coil needed then.
So Mac's out of action until I get a new coil and some carb bits for Mabel..