Thanks for the replies. It looks like there is nothing technically challenging to this upgrade. If I understand it correctly the sticks and buttons just have switches and no other electronics in them. As for the wiring, the sanwa wires are super simple. Just 4 directions and a ground. I like how simple this arcade stuff is!
As for the focusattack link, thanks for thinking along, but I am in Holland and shipping from the US is there 22 usd. I think I'll get them from
https://www.akishop.jp/ or ebay.
I scaled the two blueprints on top of each other and they seem to fit alright. I will just makes some extra holes in the sanwa plate if it doesn't:
