Do you need to replace the video switching as well? Are you upgrading to HDMI audio? The simplest solution is probably to replace the 5805 and external amp with two new AVRs. Use a receiver for the 7.1 room with a zone 2 HDMI/analog/digital output to feed the 5.1 room. You could even use a slim (possibly preamp only) receiver and keep the existing amp if there is room.

Unless the old external amp is some special piece, this should replicate all of the functionality of the existing system, depending on what remote system is in use.