Since I'm just idling at home, I thought it would be a good idea to provide some DIY advise. There are much better places to find out how to install a thru-hull (skin fitting) but I'll just go through the way I did it.
I didn't really need to replace the thru-hull but just the gate valve which looked a bit dodgy. A ball valve is much better, particularly for engine cooling water intake as it is very easy to see if it's open or closed. However, the gate valve was stuck on the thru-hull. Because of my poor tool-set and a mediocre skill-set and imagination I could not figure out some way to separate the two. So I decided to replace the whole thing. That way, I would also make sure that the new thru-hull would be up for the job (hopefully).
Initially, I chipped away at the backing block of the thru-hull. That way I would be able to pull it a bit further out under the hull, enough to cut it with an angle grinder. Unfortunately I have no angle-grinder ( I know...what's a man without an angle grinder?) so I had to borrow one from the boatyard. It was then a piece of cake to cut it off. (I won't mention my desperate attempt to cut it with my bosch multitool).
Once that was done I started working on a new backing block for the new thru-hull. Some people recommend a GRP backing block but I wasn't prepared to go into so much hassle just to save my life (OK, joking, I just don't think it's necessary if you do a good installation). So I cut a square piece of some leftover 12mm marine ply (I'm running out of that but hopefully will not need any more of it soon). I then cut a hole (about 19mm) in the centre of the block (well, to be honest it wasn't exactly in the centre but nevermind that) and then covered the whole thing with 3 or 4 layers of epoxy (letting each layer cure for about 1 hour before applying the next one). The hole should be a little bit wider than the threads of the skin fitting otherwise the epoxy layer may be compromised if the threads of the fitting dig in the block. It took a bit of trial and error before I got this right, but in the end the thru-hull could pass through the block with little contact.
After that I covered the hull side of the thru-hull with a thick layer of Sikaflex 291 and shoved it through the hull (the hole in the hull was already there remember, but if you open a new hole then it's another story).. I was alone so I had nobody to push it from the outside while I went inside to install the backing block but no harm done as the Sika kept it in place. I considered epoxy gluing the block to the hull but thought I better not in case I need to replace again in the future. So I just put another layer of Sika291 on the hull side of the backing block and pushed it over the threads of the thru-hull which now protruded through the hull. I pushed it down as hard as I could so that it made a good contact with the hull and the excess Sika was squeezed out. I then repeated the same with the flange (Sika291 on the hull side) and screwed it on the thru-hull initially by hand and then, when it was quite tight using a wrench, until I heard the block suffering under the pressure :) I took off the excess Sika around all fittings and distributed well around the block-hull surface and flange-block surface. And that was it. Now you can tell me I messed it up and I'll live in fear of drowning every time I sail.. I will post some pictures soon so that you can admire the craftsmanship!
