the easiest way to defeat a simple lock is with a similar key and some jamming. you know how a bump key works? or get a lock pick kit or, grind or drill out the four rivets amd replace with carridge bolts. after that buy a new set of locks. or get a locksmith out to make new keys for it. either way, it will cost money, long term, buying new locks is a good way to go imo, but more difficult to find so it takes more time. locksmith is pretty cheap and fast as hell.
edit... saw the above comment, if theybare numbered, then this is the way to go. cheapest and fastest.