Drive side oil seal flange

Difficult to envisage the problem.  JB Weld is pretty good.  But...like all Epoxy resins, it tends to flow out of place.  If it came unstuck, would it end up inside or outside the cases?  
You should be able to attach a picture.  Sometimes it goes wrong first time when I do it.

I used JB weld to try and fix a similar issue but when the crankcase was heated up, to fit a new main bearing, the previously hard goo just melted. 
A Low Temperature Alloy Weld kit such as Lumiweld can be used to build up a shoulder or get a pre-made shape to stay in place.
I ground a 3 thou shim to the necessary diameters and stuck this is position using some Loctite.
The oil seal also helped to keep the shim in place and appears to have worked as a fix.

All the flange does is act as a back-stop for the oil seal and there is still plenty remaining. On the 850 Mk3, the flange was removed entirely, presumably as a consequence of them setting the main bearings further apart. It makes it harder to replace them, but doesn't cause a problem in use.

I'd be inclined to simply clean up the break and examine closely for any hairline cracks. If the rest is sound, further repairs probably bring a risk of causing additional damage.