Wolfgang, not idea what it is, best guess, an early Watercraft who made odd ribs for the commerical market although seem to have disappeared lately. Do you know what length it is? You could try Tony Lee-Elliot on hot Ribs, been around since the dawn of ribbing so he might know or might even be one of his (Flatacraft). Kein Goulding the Webmaster for Hot ribs is another who might know.
The ali extension is intriguing, you must bring that along to Ribex in June would love to have a good look at it. Does the Ali exactly match the GRP hull underneath ?
Set of new tubes will be about £3000 in the UK, Eurocraft, Henshaws, or even Ribraff who regularly posts on here.
Yes I would pay £6k for the engine, hull and trailer.