The general approach is to use a combination of Subflows and Workflow Extensions as substitutes for what has been done in the legacy customization, or, rework the underlying business flow so that the legacy customization is no longer required. However some legacy customizations modify the base workflow in a way that is very divergent and moving off of the legacy customization isn’t possible while the existing functionality is maintained.
The very first step is understanding what exactly the legacy customization does that is different to the base workflow and documenting that. Do you have such available?
I’m happy to work with you further on these as required.