[PeterisP]

The core idea is that you need to imagine what would be the condition of the artifact in question if it'd be reasonable to do the required modifications, and (slowly) push the environment towards that state, filling in the gaps.

IMHO it's to be expected that you won't get where you want to be, you'll fill some of the gaps but not all of them.

The key issue is to identify what is the major pain point that prevents you from doing this. For example, in a particular similar situation for me the key points were (a) ability to reliably build a deployment package that's sure to work; (b) brief documentation about the functionality of the main components of the software and their interaction/interfaces; (c) creating a basic suite of tests to ensure that key functionality keeps working as intended if we change/rewrite certain parts fo the codebase.

The pain points will be different for you, but that's the direction that needs to be identified and taken to proceed properly.