That's sweet, what a cool engine to have gifted to you!! What oil are you running in it? It may have been designed to run a thicker oil than normal because of increased demand? Perhaps 20w50? In a car, oil actually takes a lot longer than coolant to get hot, when the temp gauge gets to normal temp the oil isn't even halfway to being up to it's own temp and when oil gets hot it gets thin, could possibly explain the loss of pressure BUT what is the pressure cold vs hot? I'd start digging around and doing research on ultralight aircraft engine mods before diving too deep.