V Spec should be a good choice. Looks like it's a four bolt, and also looks like it has a separate head button (heat sink sits on top of head button; rather than all in one heatsink and combustion chamber). Most of the four bolters have the same bolt pattern, and they're fairly common (most apart from CMB, Axe-Rossi, OPS), so a cooling jacket from a knackered engine could be a strong possibility. Also an easy item to make for anyone with a reasonable sized lathe / mill setup. Just a disc, with a big hole in the middle, bolt holes, and a groove cut into the "land" between bolt holes and plug hole; deep enough to drill into from the edge, and tap for nipples / fittings. If it's SG crank, then gearbox is preferable. The driving gear for most popular gearboxes will fit straight on a SG crank, whereas direct drive would require a custom flywheel, and probably either a custom or modified coupling too. VZ-M is the OS marine V Spec. The cooling jacket would probably fit, but may be pricey; partly cos it's OS, and partly cos it's trick; has a nifty cap in the middle, for water cooled glow plug.