Mostly unrelated story: I did a security assessment for a small bank that had moved their core processing software from the original HP 3000 hardware platform to a hardware emulator environment running on windows (something like this https://www.stromasys.com/solutions/charon-hpa/)

We cordoned it off from the network assessment out of concerns that it would be fragile. The assessment went as expected, but as we wrapped that portion the bank manager wanted us to have a go at the HP off hours ‘just to see’.

Never did figure out which part died, emulator or emulatee, but it went down hard within about three seconds.

Why not to hire a reverse engineering company and restore the source code for them?

General question: how much would one of these services cost? Any references to any?

It depends heavily on target, architecture, used programming languages, binary size, if the binaries were stripped, etc. From $1k to millions.

Would that be legal?

Australian lawyers from the top of Google Search results say that it can be if done careful:

> Generally, where a computer program is reverse engineered by copying the idea of the function presented in the program code, the original computer programmer’s copyright is not infringed. Where a computer program’s expressions in code are reproduced or adapted (including into a different computer programming language), the original computer program’s copyright is likely to be infringed. This means that where reverse engineering occurs by “clean room” design, Australian courts are unlikely to find that there has been an infringement of the original author’s copyright.

Source: https://www.dundaslawyers.com.au/reverse-engineering-of-soft...

It is is their own code, why not? In the worst case there is always clean room reverse engineering.