Firstly, it depends how experienced you are with the code at hand.

The project sounds fairly large scale, and no matter how good you are it's always going to be a big step when you dive into a strangers code.

You should allow extra time for analysis and getting to grips with the fundamentals of the CMS. Ensure you understand it and are confident you can continue work on it. Do not at any point try to bluff your way through it - be honest and put your hands up if it's not working.

Ideally you would be able to examine the source prior to any fixed agreements, so that you can determine whether you're up to the job or not.

If the CMS is not your area (whether it's too complex, a particular language etc) then take the bold decision and pass it on.