Hopefully, but I am not aware of AVgas 91 UL is it only available in UK ?, I only know use of Mogas unleaded 95 or unleaded 100 on rotax engines.
Some use a blend of mogas and avgas 100LL on old big engines, but with mixed results. Overhauls of these old Lyco's and continentals are tremendously expensive. So use of adequate fuel is advised.
It is also about time, that electronic ignition varying spark advance in relation to piston angular velocity, featuring an anti knock sensor to prevent detonation makes it debut in aviation; Perhaps with 1 magneto as a back up for redundancy. Then alcohols and benzene additives used in mogas would severy increase the vapour lock tendency on hot starts in hot temperatures and consistency of operation at higher altitudes due to thin air lack of pressure.