I would agree with Per.

If the device is your own company's product, you should know how it should behave.

Then just re-write the whole thing from the original specification.

The odd tricky piece of magic can probably be identified from your disassembly.

I have written and used intelligent disassemblers. You can identify stuff fairly easily, but it really is not worth the time. Hint: you find references to the special function registers, and name the appropriate subroutines with the equivalent C library functions.

But most importantly, you document and store your new intellectual property somewhere safe.

David.

p.s. Your boss may find it cheaper to bribe the original programmer to return the source code.