Christian,
using a bent "left hook" header per Don's suggestion would be another option for a fully inverted RE engine. However, I suspect that you will run in to "reach" issues as the thrust line on the models is higher than the header intends. Your header rear section might ride high in the fuse causing your fuel tank level to also be higher than desired if you're not using a pressurized engine such as a YS or OS. Then again, if you're going for a inverted RE setup such an engine is not a bad idea.
I'll throw out another option for a RE inverted engine - use an S-shaped header to put the exhaust up in the top deck of the fuse. The pipe runs internally and a removable light ply floor is used to separate the pipe duct from the radio gear. For such a setup it is best to put the tank as close to the CG as possible since the header needs its space in the fuse behind the FW. This is how both the Magic and Skymaster are configured. At least one pilot back in the 80's had a Tipo like this - Steve Helms. It would probably work best with the T750 or T825 than the Illusion due to the mid draft position of the wing in the latter.
David