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.