D52 and D75 are KB262 diodes, they are known as "stabistor" diodes which are made to have a very constant forward voltage regardless of current or temperature.
they are also known as "forward reference diodes".
those parts aren't made anymore and you're going to have a bit of a hard time finding them. they cross reference to a Panasonic MA27T but they will probably be almost just as hard to find.
two 1N4148 diodes in series is your best bet unless you just have to have that factory look.
as for your 148F, Nomad's advice is sage and should be taken.
the only thing i might add is that power mics tend to squeal a lot easier if the modulation limiter is removed from the radio.
the usual suspect in your radio would be TR24. if this is removed, replace it with a 2SC945 transistor as it will make SSB sound bad when gone.
you might also find R131 gone, or TR26 gone.
up to you if you want to replace them but the radio will be prone to overmodulate if they are gone.
LC
