What do you do without source code?

This happens to me quite often (I'm a Front-end developer):

The client needs something done, either some adjustment to their sites, functionality added or changed, "simple styles", but then they don't have any source code, they only have the minified version of all the files or even worst a super compressed ALL-IN-ONE file with javascript and CSS bundled in a messy code.

You have to deliver, so what do you do? Usually, I just create a new file, add the code, a lot of !important on each class, a bunch of #selectors, but that just makes it messier and unmaintainable for any future developer who has to fight the code, that includes my self sometimes.