In the end I realised that the crankshaft taper on the CG250 crankshaft is different to the crankshaft taper on the old 6v flywheel crankshaft. The CG250 CDI engine appears to have a longer crankshaft on the flywheel side. I don’t want to machine the crankshaft on the CG250 engine, so that’s it, I’ve given up and re-assembled to CG250 engine to use as is.