Curious through-hull design

The nylon elbows would worry me.  Unfortunately, the location of the through-hulls is such that you may have trouble finding a bronze or marelon replacement that will fit in the confined space.  It depends on whether you can find a tight-radius elbow. 

I think the only other answer would be to fill the holes and drill new through-hulls.

Think how long it would take to access seacocks in those locations at  sea.  It's not that much work to rip them out and glass up the holes and fit new through-hulls *with* seacocks somewhere where they can be reached without taking up the cockpit sole.  If you cant reach them, they don't get used regularly and then you find they are seized next time you want them! How far back can you comfortably reach beside your engine?

New backing blocks, Seacocks sized to the hoses  rather than trying to match the old holes (which if damaged during removal could well need considerable repair work) and *much* more room to fit them means the job will go quicker and with far less cussing. 

The only reason for deck scupper drains exiting at the waterline is to prevent water stains on the sides of boats that are neglected in some marina.  If you are using your boat often enough and looking after it, you neither need nor want them.  I'd look to see if they can be *totally* removed with the scuppers draining directly overboard.    , otherwise its seacocks in accessible locations again. or if you have to relocate them, then if you move the through-hulls  right up the sides to well above the waterline, you may well be happy with double clipped hoses, no seacocks and tapered bungs for emergency use.  If you are using any sort of plastic through hull, Make sure you have at least one bung that will fit the hole its fitted in!

I wouldn't worry much about  a small diameter, properly installed, good condition threaded drain plug on the side of the keel at the lowest point of the bilge, but if it's really on the bottom, get rid of it!   A previous owner obviously wintered the boat somewhere with a cold climate as a drain plug is usually used where there is a serious risk of frost damage.  Its very little risk unless you have a 'hot' wire drop off into the bilge and it gets eaten by electrolysis.

Ian,

The previous owner put hatches in the cockpit sole, which are easily opened. So the seacocks could be reached in a pinch (now, whether one should have loosely-gasketed plastic hatches in the cabin sole is another matter). When I haul it, I'll strategize about where to run the things instead. Hard to picture where they could go in the water.

Stefan, bronze elbows and ball valves is what I had in mind.

As for the bilge plug....I misspoke. It is indeed a garboard plug, not really on the bottom. I have no idea about its integrity. It looks bronze and well-corroded. Probably hasn't been opened in ages. I understand this boat was on the hard for years, and not sailed much. As a result, not much wear, but crates of documents on owner projects over the years...