Maybe copy it and create my own version so it won't get overridden on update? yes, other things "could" happen but I'm not planning ongoing crazy with changes. We're senior devs, I'm not particularily scared to create our own versions of certain modules... :-)

How would I go about getting the source files on an existing project?

Thank you!

Yeah, it sounds like you may want to create your own registration module. The way most of us do this is by either finding and starting with an existing open-source one or... Most commonly, create a module and then pull in copies of the registration code from DNN itself.  Then, begin making the necessary tweaks you're interested in having. 

The latter approach is what I'd recommend.  This way, you're not moving forward totally on your own, regarding security.  Meaning, that you keep the code matching what's in the core of DNN itself as much as you can. Then, as newer versions of DNN become available, you can merge in any updates you see, so that you can continue to take advantage of the updates coming in various DNN releases.  

However, there's still another option that I sometimes employ.  You can use a third-party module like Live Forms from Mandeeps to not only create a working custom registration/login form, but it can also execute other code you wish to have run.