Work with someone elses code on PHP - pull out individual blocks

Good afternoon.
There are working algorithms on php. Algorithms are deployed through Docker.
There are a number of tasks to be done on them:
Pull out certain features and formalize them into separate standalone applications, working on REST API to work out a couple of small points For how much I understand the code is high-quality. There is no documentation, but comments go through the code.
There are schemes of algorithms or their text description.

We assume three options for implementation:
Find and pull the necessary functionality Find the necessary functionality to localize it (make it independent), remove unnecessary Find the necessary functionality and localize it, unnecessary can be left.
You have a desire to poke around in someone elses code.

Terms and cost will be discussed in the course of the case.