When Phillips Petroluem did their initial engine tests with unleaded gas in the late 1970's, they found that without the lead, valve seat recession took place. They also found that serious deterioration didn't happen until the engine ran above 3,800 rpm, notably higher than the limit on the K-F 226 let alone what it would run at in the range of 60-70 mph. While hardened valve seats would be the best approach, for the time being it should not affect engine performance that much, especially if you put in any gas additive that incorporates the PowerShield (trade name) compound made by Lubrizol here in Cleveland.
If the engine was rebuilt 20 years ago and has only 1000 miles on it, You might want to tear it down to inspect the internals as lack of use can be just as damaging as high mileage