jumpwings is right, the non processed "wet" food will be the biggest hassle.
All your dry foods that you would take out of a commercial packet are fine to rewrap.
The age old problem is tinned goods, you cant open a purchased can and put the contents
into a flat sided tin and reseal it, you risk giving yourself food poisoning.
Having a straight modern packet come out of a K ration box defeats the whole purpose.
So rewrapping them is the answer as stated.
Making a period packet or wrapper is easy, all you need is you computer, printer and the paper.
A glue stick, scissors and your contents and your into it.
Basic food bags for coffee, sugar etc are the easiest.
I suggest foil lining some bags as your contents will bleed out the paper, plus it stays fresher longer
if you are going to do a heap and not use them for a while.
Thats just as easy with some alfoil and a can of spray glue.
Some I do
This is one foil lined as rice bleeds