Hagglund BV206 Tracked Vehicle power loss

We have a Hagglund BV206 tracked vehicle that is powered by a European Industrial Ford V6 (136 hp). The engine was manufactured pre-1984 and requires leaded fuel. We haven’t run leaded in it and now it tends to begin losing power once it’s been running fro 1/2 hour or so under load. Could this be due to running unleaded fuel in it? It is not a fuel/air supply problem. Are there any solutions such running a lead substitute additive or replacing the heads with ones designed for leaded fuel? These engines are similar to the Bronco II, or so I’ve heard.