If you weld it a bead at a time you're still going to have to get the same amount of heat in it every time to weld the next bead. If it was steel it wouldn't be so bad I think.
What I would be doing if it was mine: Grind a bevel on on both the case and the snapped piece. Probably more so on the broken bit than the case so your heat is concentrated more on the broken bit. Screw the bolt in. Put a wet rag over seal area. Tack the piece in place and wind out the bolt while it's still warm. Weld it quickly in one go, don't mess around using a real low setting. Run a tap through it once it's cold. If the seals are damaged from welding both the countershaft seal and the clutch detent seal can be removed without splitting the cases (just went and had a look at mine which is apart).
Another alternative is you might be able to make a guard that bolts to the top bolt and the ignition cover bolts?
