I looked into this a few years back due to a faulty Say It Again board. They have used a freely available schematic to make the new board. You can find here: http://www.bigclive.com/centaur.htm

I have never tried one, but I would guess that it works fine. It also gets past the problems of the SAD4096 by using the HT8955 chip. This chip is also obsolete, and hard to find now... I know this because I was going to make the board myself a few years back and was trying to find the HT8955. In the end, the customer I was researching this for decided he could live without the echo (the board you are referring to didn't exist then).