I have to admit my first thought would be soldering it (NOT with the Iron) just letting solder drip into and over the crack from both sides, then lightly sand it down, or grind it down with the dremel on a low setting.
Dripping the solder will most likely not allow it to adhere to the base metal it'll cool and solidify when it hits the surface. The only way to really fix it would be to solder, but you need to be VERY careful with how much heat you apply.
What I would do is this (it's very close to a method I've used on other white metal):
Get one of those tiny soldering torches that run off butane. Buy some soldering paste- the stuff that is basically ground pewter dust mixed with fat or a soldering agent (they should have it at auto body shops). Spread soldering paste over the crack and heat it with the torch CAREFULLY.
*Note: I've never tried it with soldering paste- I usually just cut a few small pieces of solder and lay them where I want them to bond. However, soldering paste is sticky goop, so it might be a little easier to work with.
What you need to do is hold the flame to the metal for a few seconds at a time and look VERY carefully at the surface the whole time. First the paste will melt but you continue heating until you see it bond with the metal of the gun. When this happens, it kinda "pops" or snaps together, like two drops of water that merge. The surface will also probably turn a little blue around the bond. When this happens remove the torch IMMEDIATELY. If you keep heating it for more than a few seconds after they bond, the whole area will melt and droop into a puddle in an INSTANT.
Final warning: This IS a risky method, and you can easily mess up and ruin the parts, but it does work and will provide the best fix.
Whatever you do, DON'T use a soldering iron (not even one of those cold solder things) because the heat will spread to much throughout the entire piece. You want to "flash heat" just the area that needs to be patched and only an open flame can produce enough heat.
