Modifying the core tracker module
It's the best to clone the module, rename it to let's say "custom_tracker" and there do the modifications you need.

As I did this few times with other modules, here are some tips:


Rename with "custom" prefix/suffix to avoid name clashes. Don't forget to rename all function and variable (I mean the stored ones) names as well as all the files (tracker.module => custom_tracker.module)
Test the module in local before using it on the live site.
As the module isn't too big, I suggest reading all code and do the renaming manually. It's better than simple replace as it might remove/screw wrong parts and also you can better understand the module.
If you would like to be able to improve the module later on, mark the changes/additions you made so later on if the module gets updated you can merge/add those improvements to your module as well.