Help Needed - Winter is coming

[beetle1303]

Hey guys, i knew there was some rust behind my dash (1303) and that was was related to the fresh air box aka fresh rust box. I took the cover off only to discover a handful of leaves and piles of dust etc, a clogged water pipe and patches of sealant everywhere.
here are the pics of my discovery
the first three are from the box and the last two are for the inside
in the last one you can see a small hole that developed after i was trying to see how bad it is...

I did a bit of research before posting on keversite.nl for wiebrand's 1303 (B xtreme B tuning), but you have removed the fresh air box. From what i understand the only solution is to cut a patch from a donor car and replace the rusted bit. My question is how difficult is it to be done, as i suspect that the bottom of the box is rusted as well. Does anybody knows which metal parts are joined together in the area show in pic 4??

Chris