JUst a followup - I used the JB Weld, it worked brilliantly, the crankacse cover is fixed.
I was so impressed, I've looked around for other things to fix. Last weekend, I repaired the glovebox lid on the Fart - both hinges were broken. Even though it's some softish plastic (PP?), the JB Weld worked brilliant;y, and the glovebox lid is actually stronger than new now.
One hinge I just glued up, the other (which was completely snapped off) I glued, wired together with 10A fusewire, then reinforced on one side with fine aluminium mesh (insect screen).
THis stuff rocks!