Wolesley WD and Ruston PT engines use a bolt on pulley, could you adapt one of those?
That looks fabricatable from a piece of laser cut plate, an offcut of thick wall pipe and some hot glue.
Even simpler could be an adaptor plate to a/the standard pulley shown above - some careful marking out of 7 holes (centre and 2 x 3 pitch circles) would work. The OD of the adaptor plate would not need to be too pretty, just functional.
Do you have a lathe? Be easy enough to make a plate to fit that, then add whatever attachment or adapters you need