Hi Enzo,
the water leak from the pump is a frequent issue.
It's quite easy fix it: you need only replace the ring on the shaft.
The water pump is the same of the later Ducati, so you wouldn't have any problem to find it.

Unfortunately, I haven't any experience with the Dellorto carbs. I cant' help you.

Ciao

Howzit
Welcome to 906 land,once your on top of it dude" she'll be right !bro".The faults are plentiful but are all easily overcome.Most parts [crank ]are shared with 907,851,916,ST2 etc [eg all water cooled models] Couple questions : How you testing for manifold air leaks ??have you set vacuum for ignition unit[assuming you got OEM Magnetti-marreli digiplex unit]
Ducs are very easy to work on if you have a good mechanical mind .Common size hex heads all over .but beware the Italians like 15mm as a bolthead size.
Cheers Andrew