Cost to replace tracks with new would be way more than the machine is worth. You might find a good set of used tracks but even then your probably gonna pay upwards of $3000 for them, on a good day. Either way you go, tracks and anything else undercarriage related is going to be expensive.
As far as hydraulic track adjusters my suggestion would be this. Open the fittings like your collapsing the track and see what happens. You might have to bump the idler with another machine to push it back, but doing so will let you know if everything is actually locked up or not. Too it should push much of the old grease out and maybe clear whatever is not allowing the new grease to be pumped back in. That said I've never run into a machine where it simply sitting makes the track adjusters not take grease. How long exactly has this machine been sitting unused? IF the fittings are actually blocked clearing them shouldn't be that big of a deal. However if the adjusters have been sitting so long that they have rusted, and will not move, then there's no telling what your getting into. I've seen them where the bore on the adusting cylinder was so rust pitted from sitting that there was no fixing them. Even new seals wouldn't seat because of the rust pits.
All of that said, a machine that old isn't worth alot to start with. With any kind of major undercarriage problems like your describing are, or potentially are present, I personally wouldn't give over scrap value for the machine regardless what shape the rest of it was in.
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