Any pictures of the plates and blades? They are probably 3/16", which are standard for these grinders. That would be the typical hamburger/sausage grind.
Unless they are terribly rusty (and even if they are), you may be able to rehab them for use.
Get some wet/dry sand paper in the 200 grit range, and you will need a flat surface to rub them on. Piece of plate glass, flat plate steel, etc. Simply rub the back of the plate that mates with the blade back and forth, round and round over the sand paper. This will brighten it up and true it up. Same with the blade. Rub the bottom part that mates with the plate back and forth to true them up. What you need to see is a dead flat, shiny surface on those blades. The leading edge of each blade needs to mate up with the plate. No gap or upturned tips at all.
Somebody (not me) attempted to sharpen this blade and ruined it:
They tried to sharpen the leading edge instead of the back. Blades now work like ski tips and ride over the meat and sinew, vs. shearing it off. This blade won't last 5 cranks of the handle before the plate balls up with sinew. (Which is why the former owner probably never used it and when I found it, was virtually like new......except for that messed up blade). The reason he probably tried to sharpen it was because he failed to screw the ring down tight. A loose ring gives the same affect as a dull blade.
As for sharping, these blades are hardened steel, and you will play hell doing much, if anything with them. If totally ruined, you might try using a belt sander. You can't do any more harm.
For our purposes, I think hardened carbon steel is fine. They don't get that much use.
If rusty, get a product called EvapORust, found in auto parts stores. It chemically binds with and removes the rust, and cleans up with soap and water. Pour some in plastic tub and dunk the plate and blade completly. Let soak about 24 hours. There may be some pitting left, but still perfectly usable (if the blades are true).
If all else fails, you can find replacement blades and plates at Bass Pro, Gander, Cabela's etc. (LEM stuff fits these) or online from a bunch of sources. Chop Rite will even sell you the originals.
But in the meantime, try to fix what you have.