They're still cheap enough to find (if you don't go IBM Model M for the keyboard) and work well enough. I actually have a PS/2 to AT keyboard converter like Puck was talking about. Works really nicely. Unfortunately my nice PS/2 keyboard with switches has some dead keys on it so it's not being used at the moment.

The only thing I swapped out on mine was I moved the hard drive on my 486 and IBM XT to compact flash cards. With the 486, you can just get a CF card slot that goes in the expansion bay and plugs right into the IDE controller. Super simple. For the XT, I built an IBM XT-IDE card that goes into the ISA slot. Definitely way more reliable than old spinning magnetic media and also easier to swap files in a pinch to newer computers.

Here's a link to the XT-IDE build: https://nediscapp.herokuapp.com/discussion.cgi?disc=46108&article=34529