> HP/UX boxes tend to be expensive, for reasons I can’t really understand

Probably:

> However, the effort to rewrite the code that runs on this machine had some surprising setbacks, and I had to wheel it back in.

You can still emulate them.

That is part of a solution but that emulation still has to run on something. Getting that emulation hardware certified for the application could be prohibitive. Are there drop-in replacements for the hardware that perform equally to the HP boxes?

If there are legal requirements for certification, then you are out of luck, but you can probably convince HPE to do a pinky promise their new boxes will perform as well as the old ones.