That wire is made of an alloy that has a certain amount of resistance. The float grounds that wire, so it's essentially working to make the wire "longer" and therefore increasing the resistance. It's the same kind of wire used in, say, a toaster because it gets hot when you put voltage into it.
On my 560SL the resistance measures from 3 Ohms when full and 69 Ohms when it reaches the reserve fuel amount. (Which is helpful to know when trying to diagnose a fault in the gauge cluster.) The copper wire you replaced it with has almost zero resistance, which is why the gauge reads full.
You can find such wire on eBay, and the data you need is the Ohms per foot, and the exact length of the wire needed. Resistance Heating Wire | eBay
Note that the "40 gauge Kanthal D resistance wire–Resistance- 84 ohm/ft 275 ohms/m" sounds about right, but you'll have to do the algebra to get it exactly right. If you have an industrial wire shop where you live, they might be more helpful in finding the right type of wire.
I'm not an electrical engineer, but my best guess is that you'll find it easier just to buy a new sender, but if you successfully pursue just replacing the wire, it would help folks if you let us know which kind of wire worked correctly.