English tends to evolve and not evolve in interesting ways... Are you trying to make your North Korean sound like an American in the fifties, or just dated?

If you really want to emphasize how old the speech is, I would suggest using thee and thou in lieu of you, or in addition to you.

Slang is something that would be easier and can change very quickly... or not. Cool has been cool since the 50s, but hot has only recently been considered cool. If you want to date your slang in a different sense, you could use something like tight, which I think is out already.

You could also try emphasizing pronounciation differences. Apparently in Shakespeare's day, the letter h wasn't aspirated at the beginning of words, so hair, heir, and air were all homonyms.

Edit: Oops. Should be post #3.