Over size head valves are not available STD to suit 6G72 Mitsubishi engines but the is alternative.
Jaguar exhaust valves part number X1571 from national parts at a cost of $4.83 + gst can be used with simple modifications. Turn the valve head down to 37mm (or desired size) kline guides to 5/16 & tip the valve end 0.020″ which still leaves plenty of meat on the valve tip ( more than original)