I haven't found JB weld in Australia yet, but there's another American product I've got a tub of called "Devcon" and it's magnificent. I was given an old Suzuki GS1000 based dragbike years ago, and the upper crankcase was busted around the front sprocket where the 630 chain let go (as incredible as that seems) and did some damage.
Anyway, using a new sprocket cover and some plasticene (silly putty) I was able to mold a piece about an inch wide and about 4 inches long to fill the gap. I then drilled and tapped it for the sprocket cover screws, and for a little added security I "helicoiled" the holes. It looked good and was still doing excellent service when I sold the bike to a guy in England a couple of years later.
I loaned the tub of Devcon to a mate and he repaired his busted crankcase on a CB750K2 where the bike had been dropped by it's PO on the points side, resulting in a big piece busted off and lost. I've also used it to repair busted off fins, and I've been told by my local powder coater that I can use it to fill a couple of scores on an alloy Katana swingarm before I have it powdercoated and fit it on my CB750 based racer special. Good stuff for sure! Cheers, Terry.