I'm going to take a brief stab at this based on existing knowledge [& hope to back it up with research later, but time is short just now;)
You can print anything you have obtained the permission to. That's actually relatively easy. Food wrappers like chocolate bars or tin cans can be printed and then manually replaced for modern equivalents.
Designs can be re-made from old drawings or even from photographs & memory. The older the product, the less accurate the representation needs to be, as there are fewer people still around who remember precisely what it was like. That makes the 80's pretty tricky, because there are many of us still around.
Sealed crisp/chip bags are going to be trickier still, if they need to contain actual product. The easiest, if not cheapest way to deal with this is to obtain [read: pay high sums for] the actual manufacturer - or even a 3rd party supplier, not all production is ever done 'in-house' - to insert some of your new wrap into their existing production run. Tricky but not impossible.
If you never need to see the bag opened from 'fresh' on camera, they could, of course, be filled with anything & glued together by hand.
A few years ago, I managed to… ermmm… 'acquire' a couple of bags of Medici Circus popcorn
At the opposite end of the scale, you can quite easily buy cans of fizzy drink labelled "Cola" or "Lemonade" with no manufacturer affiliation at all. BBC props do this all the time for scenes where an actor needs to be seen using a product. Ones deeper background are real, as they're never focussed on.