Looks like you could make an adaptor out of a Socket that fits your hex...
It'd be easier if you could find a socket that has an exact 3/4" or 1" O.D. to fit the bore of common sized clutches. Then you could machine a keyway in the socket. I cant tell if the center of your crank has a threaded retaining bolt-hole or not; but if not, you could drill and tap one to bolt the socket, and possibly also the clutch on.
There'd be some other details to work out, but let us know if you find a suitable socket and want to give it a shot. I'd suggest using a 6-point (non-chrome plated) Impact socket that's at least 2-1/2" (overall) deep, for a 3/4" bore clutch, Or at least 3" deep to fit 1" bore clutches.