Hello,
You probably have the Universal M-25 engine. That is based on a Kubota D 850 tractor engine.
Kubota parts are MUCH cheaper than Universal parts. I recently replaced the thermostat - Universal part from Torreson marine was around $60, from my local Kubota dealer it was $24. Same thing with the engine water pump. Universal was over $200, Kubota was $88.
If you do a google search you can find a parts manual that has diagrams, etc.
Good lick,
Barry