Fan might just be the Bowden cable is tight and is pulling the switch back - unscrew the little panel and see that the switch has nothing stopping it from going into max setting, if not then I guess the cable is tight - it doesn't really do anything, only opens a flap from 0 to 1st stage which is vent open then after that 2 stages of fan speed, if that cable is tight it can restrict movement of the switch.
The little bulbs are just little 12V 2W wedge (capless) clear lamps - there are two on the speedo and 1 for each gauge.
Tank - hmm - what was the excuse given that it didn't work? Did you say someone had fitted a different tank in a previous post? If so could be a mismatch of bits. It could also be as simple as needing the float arm bent down a little so that there is more travel on it into the "full zone" if it has been bent up too steep or is catching on the tank somewhere it won't let the arm swing fully past the 3/4 mark on the rheostat thus giving a false reading - dead easy to tweak if it is this, but let us know what you have first before I or someone else goes off on a tangent that doesn't even apply to you. You could also test the gauge to see if it will go to full - you can do that by bypassing the sender and connecting the wires together or decking the wire that goes to the gauge, that will in effect give full current to the gauge and it should read full - seek advise here if your not electrically minded, you are unlikely to cause any damage but lets not start causing issues!