"No package mod_jk-ap20 available" : That's probably because the jpackage repo
holds no CentOS 5 packages ?
The latest available is at "RHEL 4" : A very old Redhat 7 package.
However, a new package is easy to build from this source code SRC.RPM
Example package, CentOS 5, 32bits, from the above build : mod_jk-ap20-1.2.20-1.el5.i386.rpm
These packages are built too ( + a debug package ) :