This project sounds interesting to say the least. Not really willing to discourage you, but I'd also be kinda skeptical regarding the Zuma engine as a base for a Diesel conversion. Anyway, if you're really set into it, you should not forget that you'd probably need some sort of heater (either a glowplug or an intake "grid heater") to enhance cold-start ability.
Other aspect to consider is regarding the compression ratio: Mazda has the SkyActiv-D engine with a 14:1 compression ratio, reportedly in order to get the ignition more centralized inside the combustion chamber.
BTW I guess you have already seen something about those Evinrude spark-ignited MFE outboards used by the U.S. Department of Defense, that are also able to run on Diesel fuel for short periods.
workaround ideas to discuss among friends: Can the advantages of both spark-ignition and compression-ignition (Diesel cycle) get conciliated at the same engine platform?