In article
Ben Micklem wrote:
I'm looking for a reclaimed 1930's panelled front door (three tall below,
glazed above), and the one we've chosen is unfortunately cut for hinges and
locks on the wrong sides.

It has a mortis lock (with both ordinary bolt and deadbolt, so quite a chunk
removed from the door), and a Yale-style rim lock. The mortis is mounted at
the same level as the cross beam below the glass.

How compromised would the strength of such a door be if I were to slot a
piece of wood into where the mortis lock was (screwed in place maybe?), fill
the old rim lock hole and hinge slots, and hang on the side where the old
locks were? I was planning on having just a good rim lock on the new side
(e.g. Yale XBS2).

My front door is a similar construction and the cross-piece has a double
tenon - if yours is the same then the lock mortice probably won't have
removed much of the joint and simply gluing in a snug-fitting piece of
wood will restore most of the structure.
Similarly fill the hinge rebates by gluing in bits of wood after
cleaning up the surfaces with a chisel.