Engine runs fine but seems to seize after wot

Well, if you are sure the oils are free of contaminents, adjust the valves, and look to fuel supply, and ignition. I' d start with fuel 'cause for me it would be easier. I don' t know how high you are revving this engine, but if you could keep it below redline you could have a helper give it a quick shot of starting fluid when it tries to die. If it stays running you may have your answer.