My experience with the old tapered irons is that if they're soft, they're soft. That's all there is to it, and you'll need to replace it.
I'll give you my thoughts, which you can ignore.
If you want hard, ward double irons (tapered or not) are usually hard. If you can find one tapered, it should be good. Ward parallel irons are also generally very hard (out of half a dozen, I have one that's "normal" hardness for the era). The last time I bid on a nice ward parallel iron, it went for the equivalent of 100 pounds, so that's probably out (it was for me - I. Sorby in a set will go for a third or 40% of that).
Later norris and other R. Sorby irons abound , but they are soft, I'd avoid them for a smoother of that caliber. Later would have to be recent to guarantee hardness (hock, iles, etc).
I. Sorby and Mathieson parallel irons seem to me to be very good, but you'll have to do a little looking around to find one. The fact that you shouldn't need a cap should be helpful.
In tapered plane irons, I've found hardness as follows (in case you want to replace with another tapered iron due to cost and ease in finding):
Ward (harder than all others on a consistent basis, but once in a while, one is "normal")
Mathieson - older laminated - suitably hard, very nice. Not as hard as their parallel irons, but usually good enough for a smoother
All others - grab bag, could be anything. Later solid steel sheffield irons (pre WWII still, probably) are often softer than most early laminated irons. No clue why, but perhaps because they thought people wouldn't like to hand grind a full thickness all-steel iron. they're often thin, too, and would leave you with a large mouth gap.
A smoother is the only plane where the iron really is a lot nicer if it's hard. The rest of the planes that we have shouldn't be taking feather shavings, which is the only place harder really has it on softer for edge holding.
