I thought that you can still sell new props that need leaded fuel, is that not the case?

Note, since the 80s 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 hurdles being the necessary and not insignificant paperwork as well as concern over insurance rates.

From my understanding there was a problem with one series of engine in the seventies that was suspected to be due to unleaded fuel among the more modern product line of a major manufacturer, and while the engine was modified to fix it neither Lycoming nor Continental, the two primary piston engine manufacturers who make up the vast majority of the market, 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 and current product line.

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

Stop giving a shit about those limitations. Stop posing the question and expecting others to fix it for you. Leaded fuel is a much bigger problem than the cost of replacing or retrofitting those planes and if people don't have an incentive to change, they won't.

At the government level:

Subsidize the cost of retrofitting, set a hard deadline for no more leaded fuel, tax that fuel ridiculously starting yesterday...seriously, just invest in actual solutions instead of shrugging your fucking shoulders and saying, "but it'll cost too much."

Money ain't shit compared to public health. Give the problem a reason and the means to be solved. It really isn't that hard unless your government only cares about profits, not about improving the lives of its citizens.

Community level:

If this is your case, it'll be harder, but you need to create circumstances where either the government's or those continuing to use and produce leaded gas are punished for doing so. This is only possible through mass organizing. One of the simplest versions of this is through forming consumer unions. An even simpler method is to burn all of those little fucking planes down and burn every new one that pops up. Make it too expensive for people to buy and insurers to cover.

I think you can see where I reached the limits of my patience in writing this comment. I joke, but it is an effective means and should probably be The last resort. The point is nothing will change unless you take direct action which will involve organizing people who are affected by this problem to invoke positive change. Alone you are weak, together you are powerful. Power is what allows you to change the world.