How to make a custom Flash Player?

Question

I am currently working on a project of creating my own Flash Player to which I could implement my own custom handling and execution of SWF files that are being played in the browser. Unfortunately, the two major ones, Adobe Flash Player and Chrome's Pepper Flash are both closed source and thus I can't directly create my own versions of these. Can I however, somehow modify these, or what would be the best alternatives I could look at, that can play most flash content?