Those calipers Jochen posted look to be BG Developments out of the UK, they remake classic and race calipers. The UK in general has some really nice aftermarket integrated and stand alone handbrake calipers that actually work as they have to hold against a rolling road resistance on annual inspection.
If it were me I'd keep the existing calipers and cap the hydraulics off as a parking brake only and add aftermarket calipers to the front side of the upright. Doing that without any adapters will be tough.
The more important factors to take into account are the overall braking system, you should use a brake system calculator that has inputs like master cylinder size, pedal ratio, disc and pad size, proportioning valve impact etc. to engineer a system or you can end up adding too large a caliper on the rear that will turn the car 180 degrees in heart beat.
Fortunately Scott at SACC Restorations (vendor banner at top of page) has done all that work for you and sells a rear brake only upgrade, so why try and reinvent stopping the wheel! You can always get caliper paint and black out the manufacturer name and no one will notice when you are cruising down the Autobahn, but you will be able to stop should the sudden need arise!