Rather than a mod, this is just a suggestion/tip.
To (A) safeguard your original functions and (B) keep upgrading simple, I find it best to make a custom functions page, instead of adding every new function to the func.php.
(1) Make a php page. You can name it basically whatever you want. I named mine func_cust.php. Initially, it only needs to have <? ?>.
(2) Add any initial mods you want to add (or already have). Just put them between the <? ?>.
(3) Open your func.php, and right before the ?> (either on a line above it, or literally right before it; doesn't matter), add the following code:
Of course, if you didn't name it "func_cust", change that to whatever you named it.
Be sure both files are uploaded to the same folder.
Now, whenever you want to add a new mod, just put add it to the custom function page before the ?>
When an upgrade comes out, you just need to complete step 3 on the new func.php, and it's all up.