However, my engine is not the original 65hp VW based Revmaster engine.  I'm installing a 120hp Jabiru 3300 engine. The Jabiru engine weighs the same as the VW engine, 175lbs. 

If the aircraft's weight & balance is to be maintained the Jabiru's Center of Garvity (c.g.) must be place in the same position as the original VW's c.g. This would place the Jabiru's rubber shock mounts directly against the fire wall.

An advantage to keeping the Jabiru 3300 close to the fire wall is that it would fit nicely under the original VW cowl with only a very small (less than two inches) extension. The problem with placing the engine so close to the firewall is that carburetor and ignition system would be buried in the firewall and inaccessible. If you wanted to work on, or adjust either, you would have to remove the engine. Not Good!

From the 'access' perspective, moving the engine 6 inches forward would help a lot. However, it would move the unloaded c.g. forward, and the aircraft would be well out of c.g. when loaded with a light (150lb) pilot, a full header tank, and an empty main tank.

(more later)