I can't find them for sale. They are all 7.62.

I'm looking for the lightest most compact version.

Your going to have to have it special made, or go with a side folder, or of course the original space saver the Krinkov.

I don't know of any country that used underfolding 5.45 rifles so don't expect any parts kits.  Global Trades offered a very limited run of 5.45 underfolding rifles after the ban expired.

Your best bet would be to buy a complete underfolding stockset/trunion and install that on an existing 5.45 rifle.  An AK smith can do this or if you are a handy with tools and have a steady hand you could do it yourself.