Uh ha. Now I see where you're coming from, first thing is forget avon, they wouldn't have fitted it out, we'll start with fuel, if you have a gauge and a tank sender it's fairly simple, if you have the gauge already wired, remove the two wires from the sender end and flick them together, if the gauge moves its the sender, if it doesn't test the wire for continuity,if the gauge does move it may be that or the live or earth to it, if you just have a rats nest under the console you'll need to do this first to find the right wires, gauges and senders usually have markings on the terminals, maybe start again and rewire them
Speed is slightly different as there are different ones, some work with a paddle wheel and others work on water pressure, water one first, you're looking for a small plastic tube and it connects to the speedo, the other end is down the back of the drive somewhere, these are as useful as an ashtray on a Motorbyke and I wouldn't bother with it.
Paddle wheel, it's just what it is a paddle wheel, on the transom or under the boat somewhere, the other end plugs into the gauge and you have to put a live and earth on it.
I think I've covered most of it, most of the time they don't work because of loose connections or corrosion both are down to bad installation, if you identify the right wires and they are all black and crispy renew them, use the old wires to pull the new ones through.
If you haven't a clue what I've been talking about, I would get a sparky in to have a look, last thing you want is a loom burn out, now that does make a mess