You have the software, but not the people who wrote it.

You can no longer maintain the software or hardware.
You need to port it to modern hardware and modern programming languages.
If possible, you need something to translate the software mechanically, so that you don't have to re-engineer it. (Then when you have finished with the translator, throw it away; it's one use only.)

I can advise you on the selection of hardware and translate the software to a language that can be easily compiled on that hardware.

I have used many different machines, with widely different architectures and assemblers, and many different programming languages, high level and low.

I have implemented many different translators for a wide variety of purposes. (And, yes, I have thrown them away.)