Not sure if I have a problem.I recently rebuilt V50 MK2 gear box,did not know the history of the bike,but the casing was pretty dirty/oily and all the gears selected correctly.I treated it to new seals and a gear selector spring,the gear lever is good and positive and returns correctly.
The problem I have,is difficulty in selecting any of the gears,I have loosely assembled the rear wheel and swinging arm,just to get the gear box moving at speed.I have twiddled the adjuster on the back,but no real joy.I can just at times select between 1st neutral and 2nd.I have NOT put any oil in the box,for obvious reasons ,I just wanted to establish I have all gears, any problems I can strip the box without all the mess.
As I said,not sure if I have a problem,all I need is oil in the gear box,Im tempted to carry on building up the whole unit,and hope the gearbox just requires lube.
Input would be great,if anyone has had a similar experience. 
Most gearboxes are very difficult to change gear when sitting on a bench. They need the cogs to be rotating to help them engage. You should be able to go up and down the gears if you spin the input and output shafts, but it wont change cleanly til it’s in the bike.
I don’t have specific experience of working on a V50 gearbox, but I have had Triumph gearboxes apart and recently my Moto Morini, which has a similar selector mechanism to the Guzzi. On the bench, and particularly when dry, the gear selection can be anything but positive, however with a bit of “jiggling” I would expect to be able to select all gears albeit not very cleanly. As I say, I’m not familiar with the Guzzi box, but at the least I think I would double-check that all the gears, washers, spacers, bushes and circlips are in the right places. I know from my own experience how easy it is to make a mistake even with the diagram in front of you! If it looks good, cross your fingers, fill it with oil and see what happens! Best of luck!
bike together now,as one unit,spun the back wheel and all gears select!!!
RELlEF
A good result 