Como dizer "Achado não é roubado" em inglês

Finders keepers (loosers weepers)

Finders keepers(, losers weepers)
Prov. If you find something, you are entitled to keep it. (This is a children's rhyme and sounds childish when used by adults.)

Bill: Hey! How come you're using my fountain pen?
Fred: It's mine now. I found it on the floor—finders keepers, losers weepers.

Child: That's my hat. You can't have it.
Playmate: I found it. Finders keepers.

Fonte: http://idioms.thefreedictionary.com/finders+keepers

"Found isn't stole" poderia ser utilizado?

It can be! I mean.

Gabriel,

This isn't a very natural option in English. I think you should use ''Finders keepers, losers weepers''.