We have several very simple 120+ year old interior mortise locks that
we would lock to convert from skelaton key to thumb turn (since they
are on bathroom doors!).

I have taken the locks apart and they have very simple internal
mechanisms. It seems that I would need to replace the lever that is
activated by the skelaton key with one that has a standard square hole
in the middle so that I could insert a thumb turn.

I have previously tried adding washers to a skelaton key and putting
those behind the faceplate to keep the skelaton key in place, but it
doesn't give a good solution.

Anybody know how or whom I might use to do this conversion?
(note it is an old locks so I assume it will need some combination of
welding or custom fabrication)

The lock itself has a 2 1/2" backset with 2 1/4" between the spindle
and the keyhole.

Thanks

There are conversion kits available for this purpose.

However, another alternate would be to remove all the skeleton ket
stuff, then use a high quality wood filler, like DAP's plastic wood,
applied in small amounts at a time over several days. Once it's been
filled in, just use a regular lock install kit and paint door. At
least this way it will look like it's always been that way.

However, I am keeping my sleleton key locks on all my doors. It's
simply easier just to take them apart, clean and lube then, and get the
proper key for it, then reassemble.

writes:
There are conversion kits available for this purpose.

I have searched far-and-wide on the Internet and have not seen such
kits. Do they work on antique locks?
Can you give me a link or reference?

However, another alternate would be to remove all the skeleton ket
stuff, then use a high quality wood filler, like DAP's plastic wood,
applied in small amounts at a time over several days. Once it's been
filled in, just use a regular lock install kit and paint door. At
least this way it will look like it's always been that way.

I would prefer to keep the original antique character of the house...