A couple of months ago, I asked for advice about how to make a mutator for this (
here).
I have just uninstalled UT2004, so it reminded me I should update this post and upload the mutator in case someone is interested (feel free to improve it if you wish) :
download link To install, put the .u and .ucl files into the System folder inside your UT2004 folder. You can use the System folder located in <YourWindowsUserName>\AppData\Local\VirtualStore\Program Files (x86)\GOG.com\Unreal Tournament 2004 too. The .uc file is the source code.
You can change the respawn delay multiplier by clicking the "Configure Mutators" button when selecting mutators before a game.