I can't find a source rpm for a particular tool that I'm trying to modify. I can only get a hold of the noarch and tar.bz2. Can I modify either one of these and re-package them as a noarch.rpm?
I believe yes, take a look at http://www.rpm.org/RPM-HOWTO/ and search the web for "rpmrebuild" and -repackage flag.
with the bz2 you can do:
rpmbuild -tb packagename.tar.bz2
If that doesn't work, you'll need to unpack it (tar -jxvf packagename.tar.bz2) and edit or create the .spec file inside (packagename.spec), then repackage it (tar -cjvf packagename.tar.bz2 packagename/) and then try rpmbuild again.
Sorry if this isn't very clear, but this can get quite complex in a hurry.