'7th1st' - I don't believe I have ever seen loose Krag 'Bayonet
Catch' parts available.

There would have to be three parts: catch, spring, and button.

The button must screw onto a threaded-shaft, that is part of the catch, (like a 'nut'). Once assembled, it is likely the end of the shaft was 'peened' and finished to blend in with the button.

Possibly, the catch mechanism does not easily come apart without marring or damaging the components.

Potential source for parts, (if loose ones cannot be found): a ruined Krag bayonet.

Maybe Swiss Schmidt-Rubin or K-31 bayonet parts, if available, could be fitted to work.
(The Krag bayonet was copied from the Swiss bayonet. Swiss bayonets are so expensive now, this is a silly idea).

IMHO - As much as I like to salvage things, you would be well advised to cut your loses and buy a better Krag bayonet.