Actually I don't think the mess is THAT big. With a chisel and the vacuum cleaner present (to prevent the rust from ending up inside the tank), I was able to get rid of most loose rust. Underneath is a solid and stable bottom in which I have all confidence after some rust treatment and painting.
My mechanic drove by today (unplanned but well timed!) to pick the bike up for maintenance. He is better equipped than me and I hope he wil get the screws out.When it returns I'll do the rest of the treatment myself.