The plastic drains on the floor of the cockpit are two piece. You'll likely need backside access; usually there through a deck hatch but may need to remove to gain the access. A pair of channel locks under the deck and find a way to hold the topside piece in place. They screw apart. The drains are the least of the problem as some have noted. The primary concern is the hoses. They breakdown over the years and can literally crumble in your hands when very old. Look at the scuppers and you'll see there are inches or less of distance between the thru hulls (plastic on early boats, most now are chrome over bronze) and the waterline. The next concern is access. To get to the thru hole side on some boat models is impossible. Some may have access to one set others both. Do a thorough inspection and see what you are up against.
Ideally, you'll want new two piece chrome top, plastic/phenolic body bottoms, 2 of the best hoses you can find (GW uses marine grade with wire inside the rubber to hold the shape), and (4) SS marine clamps to hold them in place.
I've got all the parts less the hoses ready to go, but access on the Marlin is very difficult at best. Maybe I can get to them if I cut a 8" minimum hole in the sides of the euro transom and am able to reach them. Then I'd used an 8" Beckson screw plate (which I have as well). I was thinking of installing an insert in the removed end from the thru hull and pull a new hose through. Don't know if this is possible or not. I talked to my dealer and they said they let a glass guy do it. They actually cut a hole in the hull and do it from the outside of the boat... You'd think this would have been better thought out in design. I believe the hoses are installed and the thru hull side is clamped prior to the hull and liner being joined, then they do the ones in the cockpit. or do it while lowering them together. Unforutnately this isn't an option for us!
So, I've got the parts but have not worked up the nads to do it yet. Something will have to give as the hoses are not going to last forever.