I've replaced portlights
and deck hatches, and whether you are replacing an existing hatch with a bigger one, or installing a new one, is really the same job. You're going to be cutting a hole in the deck. If you're doing that in a flat and level place--no problem. If there's any curvature, you're going to need a way to match that up to the flat hatch fittings.
Check both sides carefully, make sure there are no wires running in the overhead, trace carefully and then use a jigsaw or other tool for the cut. Clean, prime, seal as needed and it is really no big deal. If the deck is the usual wood
coring of some sort, plan to clean some of that out and saturate it with epoxy
before you install anything, so that even if there's some water
leaking, it won't penetrate into the coring.