I'm not a lawyer, but is it possible for the people you got the truck from to get a lost title from WI? If it was indeed in their name and still on the WI title roster, there would only be a fee to replace the lost title, the owner signs off, and you could transfer it at the same time into your name. You would then need to pay the sales tax and title transfer fees, I don't think you need to plate it until your ready to start driving it (at least you don't in IL). My son did that on a modern truck he bought last fall. In my son's case he was transferring the title to IL, so the WI sales tax was waved, but the guy that sold it paid for the lost title fee.

I've bought vehicles before with lost titles. It was by far better to get the lost title (and I paid whatever fees were involved) and be done with it, then it is going any other way.