I'm sort of repeating what Candice posted - my husband has repaired a 6570 and used parts of another 6570 and a 6440 to do so. WARNING - if anything is stuck, DO NOT try to force it. They get stuck from the type of lubricant that was originally used, and they WILL break. And they all break in about the same places, so getting the parts can be hard because of that.
Get on the Vintage Viking Yahoo group and read before you even attempt to do much with the machine. Unless it has been serviced and they cleaned and REMOVED all the old lubricant. It usually needs to be gently heated and cleaned several times until everything is working correctly. And this it might break anyway as the parts are brittle with age.
They are wonderful machines when they are all working - but they can be a big challenge to a novice to fix = and most get broken because someone tries to force a stuck knob. We bought three machines before we got enough to get one up and running. the main cam driver almost always cracks- and costs about $50, but they are available.
I loved my 6570, and I love my 990 which is still running fine after 32 year, but it's been kept in running shape by my DH and does not have the cam problem the 6XXX series is so prone to. But I don't think I'd recommend anyone to buy one - unless they have money to burn and are willing to take the chance with it.