First question: is it leaking or was it leaking when in service?
Second question: is it a fresh water cooling system or closed loop (pressurized) cooling system?
Thoughts: if it's fresh water and not closed loop, it is not a pressurized system. That plug will likely never see 1 psi and doesn't really need freeze plugs anywho.
If it's a non-pressurized system and not leaking now, it probably never will.
IOW is it really a problem?
If it's a closed loop pressurized cooling system or shows signs of measurable leaking in the past, then yes it probably should be fixed.
Brazing or welding with niresist rod (nickel) requires significant pre and post-weld heating. It would also be a good idea to bake the block to clean the pores to get good adhesion.
Note that welding with nickel niresist is a mechanical bond like brazing, and NOT a molecular bond. it would not be as strong as new.
Point is, you might need to line bore and hone the block after welding if it warps or moves, and that makes it not worth it unless it's really rare of expensive or special.
At very least you are looking at complete dissasembly to do it right.
metal stitching is pure friggin magic, if you can find a true metal-stitch artist you will be blown away. Incredible what they can do but it's a dying art. Not cheep for good ones either.
About $200/hr is what I've paid in the past.
If it's not leaking and not going to leak, consider it problem fixed.
If it's leaked before or will leak enough to be a problem, then personally I'd look at replacement.
https://www.boatpartsdepot2019.info/w-pick/165-hp.html
165 hp Mercruiser 6 cyl GM 250 CI good used engine block: $285.99
I'd make em an offer for $200 and see if they bite.
They also have a complete engine for sale (used) for $1750
Seems high to me but what do I know?
If if you're rich: $2,399.00 plus shipping for complete remanufactured engine with warranty
https://www.michiganmotorz.com/inline-base-marine-engine-1963-p-546.html
Prolly lots more and better deals out there, I only looked for a couple minutes.