What you are seeing is not seizing. If there is no scuffing or other marks, it is not seizing. It is the engine reaching the temperature where the exhaustvalve is hot enough to start acting like a glowplug.
I take it, you have seen this on the ground? Static or bench run?

It will be a lot better in the air, but do try to keep an eye at the CHT if you have telemetry, and if possible, attempt to improve cooling by proper air ducting around the cylinder. Converting to gasoline will in cases be a compromis, albeit that in 99% of situations the endresult will be perfectly flyable. I flew my converted ASP twin under downright tropical conditions today and the engine functioned just like always