First off you need to ascertain the extent of cracking which is a very simple NDT job. MPI would be the easiest but PT (penetrant testing) would be cheap. As I think it's going up to the left also.
A very cheap way of doing a test is to put some red auto trannie fluid on it for half an hour. Wipe the surface clean then poweder on some talcum powder. The oil will blot out of the cracks. The PT mentioned above is a lot more refined but the same concept.
I am somewhat less conservative when discussing cracks. Depending on the material a proportion of stress raising occurs. If you hang 1 ton off a flat bar, you have 1 ton throughout the bar. If you have a perfectly smooth reamed hole in the same bar a stress raiser is produced and can raised the stress at that point 6 times (varies with materials). Anyway with a hole you have 6 tons. With a sharp notch you end up with upto to a million times stress raiser, hence cracks tend to grow also why on aircraft we tend to drill a hole at the end of a crack to slow down propagation.
To add another problem to what you have is like said above it does look like fatigue cracking which can occur well below the materials yield strength.
If it is easilly weldable i would machine the hole lot out and weld a new boss in there. But I am conservative
As it's low reving, low cyclic loads a cheaper option would be to, find them, full penweld them from each side and machine it smooth and re drill the holes
Putting something over it, with it still underneath will not stop the propagation.
The whole mystery about it is did the fatigue cracking occur over a forty/fifty year life span or did it occur over a shorten span as from looking at it (from the outside), it is impossible to determine the rate of propagation. If you do do a simple repair, yu may want to try a simple balance check on some rails.