There was, is and will be many debates around the unleaded vs leaded issue. Many people say that they haven't really seen a detrimental effect on older engines, even ones with cast iron seats. I can only say that I see a lot of old cars fill up with unleaded, and not many that has issues.

But back to your question, only right at the inception of the A series engines, I think they used some kind of bronze compound seat(some of the older members might be able to enlighten us with some more info here), from there on they started using steel. So unless you have a very, very old one, you don't have to worry, it'll be fine with unleaded. And no, if you would have had problems, it would have been because it was designed for leaded, and not unleaded.

The reason you might experience problems, is that the lead, other than lift the octane rating, creates a softer landing when the valves close on the seats, and for the rings on their lands. This stops the lot from wearing away at a fast rate, should they be made from softer compounds. In theory at least that is.

Anyway, if you run it with LPG, there is no cushioning effect whatsoever. Which is also why I doubt that running your older car with unleaded would have to much of an effect on it's lifespan...