I would be giving the hairy eyeball to the connections on the top. The sender might possibly have shorted internally, but it seems very unlikely. Inside, it is basically a wire-wound resistor, and a lever attached to the float arm shorts out part of the resistor.
But the terminal on the top (at least on earlier units) is just a screw through a hole with a phenolic or nylon "top hat" bushing to keep it from contacting the sides of the hole. If the bushing were broken or absent, the screw or nut could be shorting to the mounting plate.
If that isn't it, there is at least some chance you could open it up, find and fix the problem. It is a very simple device.
Sorry, no recommendation for a replacement. I've never had to replace one.