Are the screws stripped? Or, the holes in thermostat housing (more likely)?
Replaced stripped screws with stainless steel one, use anti-sieze. If threads stripped, use a helicoil kit and SS screws. Or, retap for next larger screws, get two SS screws to fit. . In a pinch, drill thru holes, use longer screws and nuts on back side (if this is possible.) Might need to get new thermo housing and base gasket, however it is mounted.
EDIT: Are screw heads rounded off? Use plenty of penetrating oil, tap, try to turn them CW a bit with ViseGrips . Best to let it soak overnight, then remove with ViseGrips. Or, drill hole and use an EZ Out. .