Cast aluminum in a high stress application is not very amenable to repairing, weld or otherwise. Forged/billet, possibly, but without a full post-heat treating process, the original strength will not be regained, albiet that the resultant strength may be adequate for a reasonably long second life.
I would attribute the Sampson crank failures to a flawed marketing & delivery strategy. I.e., given the sensitivity of the design to installation torque, they erred when selling in a manner that permitted installation by lay end customers. They probably should have required installation by authorized dealers only. Probably hard to control this, especially after initial installation, so really they were out in left field putting out a product with such a narrow design margin.