Is it like the Traction avant eg. the drum is on a taper with a woodruff key?
Have you tried heat? Get it hot with a good blowlamp, may just do the trick
On the Traction some come off quite easily but others required the proper Citroen puller, which is a serios bit of kit.
If the heat trick doesent work then I think you may find the club may hire out the tools you need, I know the Traction owners club do.