have you considered adding an inverted oil system? The Mudry Cap10s that I used to fly had inverted oil system on the IO-360 (i think it was) and you could fly inverted and do any negative g maneuvers that you wanted, well until you reached the g limits which were +/- 6.
you said that the plane is capable of loops and rolls but is not certified for aerobatics? I don't know your aviation background so I would hate to insult you or your project but that statement makes me nervous.
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