Yes, I just had a look and the manual states D57 is a 1SV35 silicon diode and it's likely to be a PIN diode based on a quick google. I couldn't find a datasheet though.
A cheap (sub $1) and available equivalent might be the Vishay BA479G PIN diode. This has an ideality factor close to 1 so it should have a resistance of 26/Ibias(mA).
Mouser have loads of them in stock at about 35p each here in the UK and the USA.
https://www.mouser.co.uk/datasheet/2...9g-1767857.pdf
As R154 (27k) appears to bias D57 with a minimum current of 0.5mA then the resistance using a BA479G will be 52 ohms. See the BA479G datasheet graph below.
If this gives too much gain on the lowest setting of the gain control then I suppose the 27k resistor could be increased to 33k and then retry the circuit. This assumes D57 has failed open circuit and needs to be replaced.
I don't think the op amp can quite reverse bias D57 if something has gone wrong with the opamp section and it slews to the positive supply rail. However, just to make sure I'd check the opamp/Q21 just to make sure it isn't slewing high enough to start to rob D57 of current.