If you are just trying to improve braking, make those improvements to the front brake with a larger diameter disc from the ST1100 Pan European and a brake adapter. The rear drum brake is plenty strong enough to lock the rear end if doing brake turns off-road and strong enough to stop the rear when both brakes are used correctly on the street. The only reason for using the rear disc is if you are trying to increase rear suspension travel with an RD03/04 swingarm and longer rear shock etc. Then the rear disc gives you the flexibility to accomplish this without the restriction of the solid brake rod operating the drum brake that gets geometrically confused when the rear travel increases. It’s a lot of expense and trouble for no real increase in braking performance.