I think it will all come down to how much you trust the current shop and what they are telling you. It is not uncommon, especially after having a couple of shops work on it, to see other issues show up.
The other issue is doing a Google search similar trucks are being listed anywhere from $6500.00 to $12,000.00 so do you really want to invest another $11,000.00 into it?
Assuming that you have only had this one company look at the engine noise before dropping or scrapping the truck I would bring in at least 1 other company to look at it maybe going as far as asking your local Hyster dealer.