You need Kickstart 3.1 v40.63.
It is implemented either with a MASK ROM or a 27C400 EPROM.
Yes U13.
If you made your own Kickstart ROM you could use a 27C800 EPROM and put inside both Kickstart 1.3 v34.5 and Kickstart 3.1 v40.63 and have a simple 3-pole switch.
http://eab.abime.net/showthread.php?t=46636