Awesome find even encrusted! Nothing wrong with nickels especially those!
Hendo0601-ketchup can be used to clean nickels, it's an mild acid. Try putting a dirty modern nickel in it for a few hours or over night, but don't clean a valuable coin. The coins we pull out of the ground are not spendable a lot of times because of the crud on them. I have used lemon juice on my clad quarters you have to watch it and not leave them in too long, some of mine turned pink. Lemon juice worked great on yellow gold!