No great tips other than hitting the drum with a hammer and using a screwdriver to pry the drum off. I suspect you are correct about the corroded wheel cylinder, that is the usual problem with them. To help prevent it in the future you can put some red rubber grease inside the boot and change the brake fluid regularly.
There are several rear cylinder part numbers in the Alternative Parts List at the top of this forum in Resources, they all fit and are far cheaper than Morgan. If you want the option of making the rear brake drum more easily removable in the future, you can fit a shoe adjuster, rather than the fixed pivot currently fitted, again detailed in the APL.
