Hello Steve,
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.
Regards, Stanley