step 7 program recreation

I've got a task to make a new, more stable PLC program, based on the solution that was made a long time ago.

The problem is that this old program is very complicated and badly commented. It is really hard understand the goals of each block of the program.
Are there any general approach to this kind of task? Are there any tools that help to understand those programs?

Those, who made the original program is, well, unreachable now, so I can't ask them for help.

Electrical docs, connection plans, talking to people who work with the machine.
If You have knowlege on what the program is doing and components that are connected to PLC, just pen&paper and figure it out
Original program is just used to confirm Your assumptions.