Some Issues Looking 4 Help

[YeA 20sF]

So the more i use my boat the more I need to fix and mantenace. I have a few new issues that I am looking for some advice on see the best way to go about fixing them.

1. Fuel gauge.. Not such a big deal but I would like to know how much gas I got. A few weeks ago I went down to the keys and I put about 160 bucks of fuel. That was a little over 30 something gallons gauge read E. The spec sheet says the 20SF should have a 40 gallon fuel tank from factory. Well I ran out of gas and quite fast i might add, any suggestions on where to start ?

2. Lower unit missing a screw..... I changed the water pump before going down to the keys and put in a new fin zinc (the one over the prop) well it seams that they have vanished along with the screw the goes upward holding up the lower unit. What type of screw do I need to replace that ? Size ?

3. My float switch for the bilge pump is dead..... How do I replace that ? just pull it from its base remove whats there and attach a new one with what ?