Do you know who marinised this engine
? The model DM12 is not a Kubota engine model. I suspect that the company who marinised the basic engine gave it this model number. If it is a Kubota base engine I suggest that you get the actual Kubota engine serial
number. On the small 2 cylinder engines it is stamped into the flat surface next to the injection pump. You may need to carefully use a piece of sand paper to get the numbers to appear as on most old engines, a bit of rust and some paint
obscure the numbers. The number sequence will start Z400----- or Z500-----. Once you have this, then you will be able to get a manual from Kubota.