Original plans allowed for use of O-200
Van's original RV-3 plans allowed for a Lycoming engine or a Continentel O-200 configuration. The plans modified the cockpit seat pan and engine mounts (not only mount, but dimension changes) if I remember correctly.
There have been several O-200 powered -3's built, but I have not actually seen any of them first hand. I have a picture and an RVator wright up of one if you are interested.
If you are interested in seeing the modified plans for the lighter O-200 engine let me know. It would most likely have to have further mods for the Jabiru but would give you some known starting points.