Note, since the 70s the vast, vast majority of piston driven aircraft engines have been able to operate on unleaded fuel. We know this because for decades GA pilots have been filling out the paperwork for an experimental fuel variance and then running these engines unmodified on the cheaper unleaded they got from the gas station down the street without any apparent issue or rise in engine maintenance/failures among pilots that do this. The main hurdle being the necessary and not insignificant paperwork and concern over insurance rates.

From my understanding there was a problem with one series of engine in the sixties that was suspected to be due to unleaded fuel, and while the engine was modified to fix it neither Lycoming nor Continental, the two primary piston engine manufacturer, saw significant pressure to drop the official recommendation for unleaded until relatively recently.

Since the US finally started to get serious about phasing out leaded avgas in the 2010s, and the aditude of its been fine so far so why risk any change has run up against said pressure, both have to my knowledge dropped the requirement retroactively with no modification necessary for the majority of their historical product line.

You might need to re-engine or more likely just get an exemption for flying historical aircraft, but the benefit to the hundreds of thousands that live near GA airports in terms of reduced miscarriages and damage to children’s nervous systems far outweighs the nebulous cost of switching the default form of avgas.