was the tank full of gas when they parked it or not? Just fill the tank with E85 like Rusty says up to where the dried on stuff is. fasten a small tank somewhere and hook that up to the carb and then you can drive the tractor around and the sloshing of the E85 in the tank will wash the crap off the side wall of the tank and when it is all loose you can drain the tank, and steam clean it and you should be good to go. It may take a month or more to get it all loose. Just be patient and it will work out just fine.