David I have a couple of a15 engines here that can be utilised. I also have a ca20 spec 71b box so pics can be taken of all relevant meeting faces. With a few bits of wood etc, we could also 'dummy' mount the motor and box together and centre it (within a few mm or so) and take some pics to show the overlap etc.

I've also got a bunch of paper/thick glossy cardboard that could be used to trace the various items (or with judicious use of a ball peen hammer, hit around the corners which will highlight the exact sizes, and could be used to start the adapter plate measuring/design.