Since not all motors are the same bearer spacing you could "hack the nose off" build up a bulkhead, use some type of threaded anchor such as T nuts, then use a spacer plate to mount a plastic or metal bulkhead mount, such as the RC guys typically use. Engine change, take the whole mounting system off, put on new engine.
It might work out that enough mounts have the same hole spacing, but I don't think they do in my limited experience.
This could allow a Fox to McCoy type of change, instead of having to stay with a fixed case width.
Phil