First thing: is it a fixed keel or a swing keel? Cat 25's came both ways as I recall
. The swing keels were cast iron (at least on the earlier ones) and as such are far more likely to suffer a crack than cast lead, which is very malleable and unlikely to crack.
If iron, and badly cracked, replacement is likely a better choice than welding. If the cracking is superficial, one might well ignore it... well, perhaps ignore after sealing it up with some sort of goop. If everything else is ok, then it sounds like something worth investigating.
Chers and good luck
Jim and Ann s/v Insatiable II lying Cairns, Qld Oz