[tialaramex]

It may also be used for some very old cars. "This should be in a museum" (and maybe is) level cars may have been impossible to adapt to unleaded gasoline. So, one option is you just don't run them - after all these are very old cars they're both unsafe and inefficient by modern standards with few creature comforts. But if you're exhibiting cars in working condition you've got two practical options:

1. Add a lead additive, you don't need very much, and since this isn't exactly your daily driver it's not so terribly inconvenient, you can buy this over the Internet. You just pour a measured ammount in to the tank each time you re-fuel. I don't think this is illegal in the US, but I don't know if it's common. At population scale it isn't a big nuisance.

2. Choose a substitute additive, which is likewise added to fuel. There are several, but whether they work for you is a question and of course the manufacturers are not interested in insuring your potentially unique 100+ year old car for damage from their cheap additive product. If you suffer mechanical problems that's your problem.

There's a lot more piston plane flight than there are people still driving a classic era car on any particular day.