sroberts, welcome to
RP.
and welcome to my world

i've worked on worse. and i've worked on tintypes. throw away all your filters and actions and global fix type tools. roll up your sleeves and prepare for a sistine chapel type restoration. it's the only way i've found that works well.
actually, you will use a couple global type tools. in fact, one of the first things you want is to bring out the detail more. and that generally means contrasting the lights to the darks more. and that means things like curves, levels, brightness/contrast and any other tools you might have like that. i've even resorted to the 'arithmetic' filter at times.
when you've brought out the detail by contrast, you simply start cloning. but it's cloning on a level that most probably never use. my opacity on these never goes over about 60% and that only rarely. the brush size is always small to tiny. hardness or brush edges never more than 80%. sometimes i even turn down the density. you're going to be doing a lot of meticulous work here. it's not quite bit by bit, but sometimes it's going to seem that way. you work from every direction towards the target area and overlap a lot. you ALWAYS work on a blank layer and set your brush to 'use all layers'.
also, it's a VERY good idea to enlarge the image on these. yours is currently 523 x 800. i'd double that at least. you also want to work in rgb 8 bit per channel, not grayscale! grayscale limits what you can use too much.
you also want to be something of an artist. you often have to re-build missing detail. you dont need to be a top-notch one, necessarily, but you do want a smattering of facial structure, shadows and lighting and so on.
the main trick is in NOT destroying the limited detail that is there while at the same time removing the damage and then re-building what's missing, if possible. if it isnt possible, dont fool with it. just leave that part alone.
as for having a physical conservator look at it with the idea of cleaning it up, that's up to you and your client. just make sure you find one that knows what they're doing. tintypes are a special class of photo. and make sure you have as good a scan as you can before sending it off to someone.
it looks like an interesting job. i may work on it a bit. i actually enjoy these