I don't think the IE2 is actually certified to run on MOGAS, 'unleaded' yes, but they are not the same thing.
If you try and run an IO540 on MOGAS, yes it would probably run fine on a cold day, fully rich. There are probsbly plenty of people that have tried.
But if you want to run it consistently, on a hot day, at high power settings, leaned out.
it won't run fine.
You misunderstand the idea behind octane rating, high octane fuel is specifically to reduce detonation, that is it basically 'less flammable' or rather 'less explosive' but it happens
If you try and put MOGAS in an IO540 and run it at full power, it may produce the same power for a little bit before detonating, because of the lower octane rating and the high specific energy it will not run.
Go and ask continental or lycoming.
I should have said, 'in the real world, an IO540 won't produce the same power with the same amount of MOGAS as it would on AVGAS without detonating.
If you look at the KW/L or NM/L figures of a lycoming or conti and compare them to even a modest non turbo passenger car engine, you will find a lyc or conti produces less than half.
The only time a conti or lyc has an advantage is in power to weight ratio, this is only really because of how simple they are.
I'm not saying an auto engine is appropriate for an aircraft.
At the end of the day,