Allodium, thanks. That second link is helpful.
I have looked at the ICs and one by one:
5534. This is an old IC but after looking at posts on audiophile forums (commonly used as an audio pre-amp) about potential replacements, there seems little reason to change it. I note that it has been upgraded between the GIF schematic and the PDF. In the GIF it is a UA709 so it was already upgraded to this low noise part.
TL064 and TL074. Equally these are pretty good op amps and I can't see a reason to change them.
4538 is CMOS and digital. I wouldn't expect expect changing it would make difference to noise.
The 78L05 and 79L05 are still workhorses and probably don't warrant changing.
The 7912 might be changed for a lower dropout regulator- might extend battery life.
All the parts seem commonly available and that is a bonus for people making this though.
Without a major redesign (Goldigger is right that there are now simpler ways to generate +-5 volt rails) the main change suggested to the ICs is to use the 7555 timer to replace the 555s as this may reduce switching noise on the supply and will reduce power consumption.
Changing/ upgrading the op amps probably won't change performance too much but I am open to wiser opinions than mine. Any suggestions/ opinions?
Using good quality capacitors is probably a good upgrade.
So that leaves the 10uS pulse delay, Minelab connector, DD search coil and the better potentiometer to work out.
Hope this all helps.
Chudster
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