So what are the possibilities to install an HD motor into the "five-speeder", using a properly machined adapter plate? \
I do not believe an adaptor plate to the transmission would be required to install a H-D engine, only custom engine mounts. As the primary drive interface of the x-wedge crankcase is compatible with H-D primaries, the H-D engine should bolt up to the Morgan motor to gearbox adaptor housing.