Almost any silicon diode can be used to replace the 1N3754 bias diode in there, but getting it to sense the heating of the output transistors is the real problem.
The diode is mounted to the heatsink so that when the output transistors get hot it will lower the bias current to the transistors. I've tried epoxying a small signal diode into a piece of 1/4" aluminum tubing as a replacement. I've also mounted a small two lug terminal strip to the heatsink and mounted a glass bodied diode so that it sat on the flat surface of the heatsink.
Anything that you can think of will probably work as long as you watch the polarity.