Dan,
You have an earlier Adventure and may have a warped opening. It's no big deal, but check it out with something flat to see where the high and low spots are. If so, I'd recommend sealing the gap with a generous bead of silicon caulk when you install you new hatch. If it gets outside the joint, just wipe it up.
You will re-use your 6(?) screws plus whatever Matt says the other two are.
Finally, part of your leakage is probably coming from your drive as it swishes water into the hull and right up to your forward hatch where it's probably been leaking in. You can block this by crafting a "swish blocker" from a foam block (the flexible kind that doesn't crumble). It would look like this:
Original gap:
Block taped on rear of drive:
Block installed:
Crude but effective! Note it stops short of touching the drive cables. Also, remember that extra speed you were looking for? This will give you almost .1 MPH at cruise speed -- a respectable start on your goal of .5MPH -- and for no extra effort!
Hobie should consider coming out with something like this.
