I do agree with NZer, D55 to D56 turbo is a direct swap. 4D55 turbo would've been an ideal swap but its hard to find used D55 turbos in the market.Even if you do, im sure thats not the best nowadays.
If you opt for a 4D56 non turbo or a non intercooled, things are very straight forward. But if the motor is 4D56 watercooled or intercooled turbo, we are talking about a bit of extra bucks spent and few more man hours of labor involved.
Whatever it is 56 is manytimes better than the old 55 and no extra mods required. I have done that swap once and she is running very healthy now.Go for it mate. Good luck!
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