As has already been suggested, I'd start with a compression test.
A bit of corrosion, or a stuck ring after 6 months of inactivity could be the problem. If the compression test is good, AND the injectors are good (which might be different than "new"), then the next place to look is the injection pump.
If the compression test is bad... then you have a decision to make as to how far into the engine you dig to try to solve it. Might be valves (not too expensive) might be rings or cylinder walls (much more expensive).
La Parguera, Puerto Rico