Repairing them is very tricky as, while you could solder or weld the crack you've lost the hardness and introduced some very localised heat stress too as well as altering the properties of the steel crystaline structure.
Then you have to reheat to harden the dies, which often involves long heat periods and very controled temperature, as well as cooling by quenching which is extremely stressfull and the repair will react differently to the actual steel too.
Finally you have to temper the steel to soften it and relieve stresses from the hardening as it's usually too hard and brittle to survive long.
The modern approach can use laser welding or the use of specialised resins or even electroplating to repair.
With the advent of cnc and spark erosion it often quicker, cheaper or less hassle to make a new die than repair them .
I think the usual pattern was a reinforced frame around the die and use untill it broke entirely or the production run was finished, then make a new die. The period literature doesn't have much info about repairs and mostly warns of the dangers involved (shattering at 30 tons of pressure is interesting to say the least- think hand-grenade).
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