Could I suggest that as AdrianR has noticed that the Mk2 version has a stronger casting and Kettrinboy has had to reinforce his when repairing it after his cracked, that any repair you contemplate such as welding or metal stitching that will leave the part as good as new or nearly so may not be adequate as the are indications to me that a design fault was recognised and corrected in the Mk 2 design?
Certainly adhesives, as good as these are today would probably not be a good choice in my opinion.
I think the only way to make the machine usable in such a way that it stays usable would be to remake the part, or have it remade as John Baron has already suggested.
A block of steel would perhaps be ideal and an improvement on the original, although cast iron is also available.
If it is of any help here is the drawing from the original Westbury article, but you would need to see if this part changed when the design was produced commercially as the Dore-Westbury. At a casual glance it appears not to have been, but please check.

Edited By Nick Clarke 3 on 28/05/2020 08:50:37