Sorry for answering in English, but for me reading Dutch is easier than writing.
I have seen this problem on quite a few Flaminias. The old cork float used to be coated to prevent from soaking with gasoline. Today they are often cracked and do not float but submerge. IMO making a floater out of cork will not last too long and as Huib has said you do not want cork crumble in the system.
My solution is to buy a complete level indicator, maybe one for boat tanks or chinese scooters or so. You can find them on ebay for about 13 Euro. The only thing needed is the floater and maybe the holding wire arm.
Connect the floater to the original arm of the old level indicator, bend it in position if required and you´re through with it.
You can see some pics here(scroll down a little bit):
http://www.lanciaclubdeutschland.de/fla ... try=20&f=3 The indicator shown is a Touring version, but things work the same on the PF
Hubert