As we know, aluminum shrinks more that steel. Therefore, if you can remove the engine mount from the plane, you can put the engine and mount in the freezer for several hours. The aluminum will shrink and the steel bolt will remain the same size. This should pull the aluminum away from the steel bolt loosening the bolt.
Another method would be to use a torch on the aluminum to heat it. Aluminum will expand more than the steel.
Either way, the bolt will loosen.
Frank