[HoosierDaddy]

I had the 555 which is similar. Generally the thermostat will install between the upper radiator hose and the engine. Your 903 may have a thermostat in each hose. It will mount inside the housing that bolts to the head. An engine manual would show exactly where. Manuals are a good investment and can be bought on ebay. While you have the system apart you should drain and replace the coolant if you don't know when it was last changed. These engines have "wet" cylinder liners that are subject to coolant cavitation corrosion. A cavitation inhibitor MUST be added to the coolant. Cooling system failures are one of the most common causes of engine failure.