There have been a number of reported camshaft failures on listeroid twins, and I suspect what 32coupe has found might account for some of them. Some steel shim stock (mcmaster.com) Weld-Bonded (steel filled epoxy) in place might suffice, but I'd also be concerned about camshaft alignment. In my admittedly poor memory, there were 3 bearings for the twin camshaft, and I'm thinking - what are the chances that they are located accurately, in line with each other, given Rajkot typical practice? If they are not aligned, then the constant flexing of the camshaft with each revolution over time will cause it to fail at some weakest point. Which is pretty much what the broken twin camshaft pictures I recall have shown. Somehow, I'd want to check the alignment before shimming anything, as one might wish to use the loose tolerance to correct for misalignment.
Dave (XYZer) has already shown us years ago that case boring of critical locations (cam, idler, crank) for listeroids are way off spec, and vary considerably. This was the underlying cause of idler gear failures. I wonder if the same underlying cause might be the problem with twin camshaft failures.
Just speculation...