There is one screw in the front that holds on the fixed contact, terminal strips, and movable contact retainer. Make a sketch of how it all goes together, then remove that screw and take all the parts off. This will allow you to get to both contacts to polish them. They are silver so do not be to aggressive. You can use solder wick to take off excess solder and old wires from contact strips to get ready solder new wires back on. This also exposes the bottom side of pole piece. If you are thinking about putting a thin piece of tape on the pole piece to reduce the chance of the relay not opening, this the time to do it. Also a good time to clean the hinge up as best as you can. The hinge is a place where poor contact can prevent the whistle from blowing. Now reassemble everything as you found it. Make sure the bosses on the fiber insulators get into the proper holes in the metal parts. Be really careful with the coil wires. If ones breaks off, it might be hard to fix. With two or three C or D cells in series, the whistle relay should operate . Use a meter or light bulb to check continuity through the hinge and contact when the relay closes.
There are several different types of whistle relay. One with spring on back is for GG1. There are relays with different numbers of contact strips and insulators on front. If relay attaches to a plastic whistle you need extra contact strip as ground is not brought into the relay with the mounting screw. It needs a ground wire, thus the extra contact strip.
It is all easily done, just takes a little time. Try to avoid relays with rust on them.