I cannot believe I am actually saying this, but I think your coin is a
good candidate for cleaning with something like
Gringott's number 1 -- the weakest solution. Even better would be the new
Conservator Mix that
Bruce cooked up (PM for source). I've got a dozen problem coins soaking in it right now myself.
I would normally go after the crusty greenies with a scalpel, but in this case, with the potential
silvering in and around them, I'd prefer to avoid getting too mechanical. This means getting chemical. Distilled water will do it also. It will just take a lot, lot (LOT) longer.
Also, the brown dirt will probably be handled by whatever you use to get rid of the green encrustations. If not, you can eventually
pick it out the the crevices later.
Mark