that's not the t stat on your motor it under that center spot where the 2 heads pipes tie together in the top of the block. just take that cover off and clean up well and either buy a new rubber gasket and reinstall or just clean up old one if it looks good just coat both sides with gasket sealer and reinstall. if you use sealant give it over night before testing but new gasket just hook to hose and crank her up.
here's your t stat only has 1 in those motors in later yrs to be little move even on head temps.
