I am not familiar with your particular needs, and the question doesn't exactly define them. But a good startingpoint with this type of problem is to factor all the stuff that doesn't change into a base class (or module), and then sub-class it to add on the layers that tweak it to work with varying needs. The sub-classes should use a uniform interface, so that one sub-class can be a drop-in replacement for another. That way, when you get a new client, with a new need, you simply subclass the base class providing the individual functionality that the new client needs, without wrecking everything else.

This is one of those things that object oriented programming is for.