To repair/prevent such a crack it should be possible to shape up a strip of stainless steel to as close a fit to the casting edge profile as possible extending well in front and below the hole, (as seen in Erik’s photograph), the strip to be fixed to the casting with a pair of suitable screws at each end. It would be advantageous if the edge of the casting and the mating side of the strap could be sand blasted clean just before final fitting and a layer of slow setting Araldite introduced to fill all the gaps. If the strap is made a substantial thickness, say 2mm, then it should add sufficient strength even if the casting is already cracked without being visually intrusive. It also removes the need to find a proficient welder to do a repair.
If there isn’t sufficient room for the snail cam how about replacing the rear washer with a quadrant of the same thickness? There is even a decent area of flat on the casting shown in the pictures ahead of the rear hole that could have been made for the job! The idea is that the quadrant is made with a set of close spaced holes set on progressively increasing radii relative to the bolt hole, ideally a difference in max/min radius of half a chain link is to be aimed at. The casting would have to be drilled and tapped for a suitably placed bolt hole, the bolt need not be large, ¼in. (6mm) should be plenty strong enough as it is in shear. So that’s job done without the need of more than a vice, drill and file I reckon. The only down side I can see is that the adjustment is not progressive as it is with the snail but I don’t think small steps would be a problem in this instance. I’d include a drawing but I can’t get my head around the doing of it! I can manage e-mail if anyone is really keen and can’t understand my above ramblings.