Good luck Rolf.
I note that the Spare Parts manual indicates that they changed the 5th gear cogs in 2005. You could possibly substitute the later pair with more teeth. I bought a 32212011 from Stein Dinse for some E15 a couple of years back. Didn't get the matching 5th gear - not yet anyway - it's cost exceeded the price of a secondhand gearbox. I search the guzzi parts people all over the world trying to find gears at reasonable prices, and still one to find. Do note that the V7 boxes will fit perfectly, at least the 5 speed ones, don't know about the 6 speed versions.
Fettle well, & let us know how you go.
Mal