If you cannot find any original decals, there are sign shops everywhere that can cut stick-on vinyl letters to match what you need. Take them the measurements and a picture so they can match it. They are inexpensive, and last years.
I purchased my 540 combine last Sept. from a wheat research center in western Kansas. I am the second owner and plan to use the machine to cut fescue seed next June here in central Missouri. My machine has a 318 cu in 4 cylinder Perkins desiel and has a 16 foot header. That is a big header around here. I have had several MF 300 combines previously so this is a big step up for me and I'm looking forward to working it.