If there's a build.xml among the mix of files, then the project is set up to be built by 'ant'.
Dig through it looking for something that looks like <javac ...>
It'll be nested in something that looks like <target name="something">. That something is the option you'll want to give to ant on the command line. Use that to tell if your fix compiles.
Then look at the other <target>s. One of them will build the .war file. Or, if you're lucky, there'll be an action that builds everything and deploys.
And yeah, back *everything* up first.