Don't know which model engine you want to put in or what you want to use it for. But I suggest you research it carefully, power and torque figures and curves that is. Experience of small Merc diesel engines is that they are car derived, meaning that although they have the power and torque it is not necessarily where you want it for an off-roader, though looked after they live to a ripe old age.
Didn't intend to pore water on your fireworks, but just thought I'd share my experiences.
Regards
Stuart :)
