When I fit a larger or smaller to an existing installation I rotate the new motor about 45 degrees and drill fresh mounting holes for the new motor.
The leads just need to be re-routed to the ESC but it is no big deal.
If after the testing I want to swap the motor back to the original I have not destroyed or enlarged the previous mounting holes.
Another method for those "Test bed" planes is to make up a second, "dummy", engine bulkhead which I drill to suit the new motor and use another set of screws to screw the dummy to the original bulkhead. In this case the normal mounting hole area of the original bulkhead had large holes drilled in the normal mounting hole position and the screws used to attach the second bulkhead are screwed into the 45 degree , (normally unused), positions in the bulkheads. With this method I sometimes use small sapcers between the bulkheads to help with the balance.
Finally the best approach is to have a number of different power pods with each being drilled to suit an individual motor mount.
Just what works for me!
Have fun!