To the best of my knowledge, modifying an engine to fit in the rear of a coupe hasn't been done. I believe its in large part to dimensional constraints of the trunk opening. Access to the top of the engine would be very difficult as it would end up under the rear glass, which would cause a number of difficulties. It appears that all the structural modifications would become quite difficult with the lack of access from above, unless you'd remove the rear deck altogether and re-stabilize the area and glass using other methods. Hence, why its been done in hatchbacks, having wide open space above the would-be engine location. Such a project is possible but would require a lot of time, money, skill, and desire. If you decide to take on the monstrous project, I'm sure you'd have a lot of interested spectators.
