Aviation engines don't need the lead for valves and seats - that's an old wives tale. All the lead is for is to increase knock margins, nothing more nothing less. Virtually all of the engines fitted to the fleet will make their rated lifetime on the new 91UL unleaded avgas. The problem engines are really the larger turbosupercharged engines fitted to larger piston powered aircraft.
The majority of the general aviation piston fleet could all run on unleaded fuel tomorrow if:
- a: it was available
- b: the regulator allowed it
Over here in Europe 91UL (unleaded avgas) is now available and has been for a couple of years, but the process to approve every airframe+engine combination (it's not enough that a Lycoming O-320-B2B is approved, the combination of Lycoming O-320-B2B and every single type of aircraft it is fitted to must be approved, except in the case of Permit-to-Fly aircraft (i.e. antiques and homebuilts) where just the engine needs to be approved. Unleaded avgas can't come soon enough, leaded avgas is actually BAD for most of the fleet, it causes spark plug fouling and other unwanted effects (as well as being very toxic). Unfortunately 91UL isn't very widespread yet, I've yet to see it for sale anywhere despite the list of engines and airframes approved to use it getting longer each month.