I am not positive if I understand you exactly but I THINK I do ...here is what I think you need to do...

1 get a long board
2 strap your fuse down to it secure..use some pads make sure it can't move...
3 take a small board and screw it down in front of the plane with two screws.
4 put a nail or screw in that board so that it just touches the center of your crankshaft.
5 now you can remove the small board, and replace it later in the same spot using the same holes.
6 remove motor from plane without fuse shifting on main board.
7 install cowl so that it centers on the nail, once small board is repositioned...
8 be careful not to position cowl too far out...

Hope this helps, I think this is what you were wanting to know?