One method to try
Luca;
What I did what to use a scrap piece of soft aluminum tubing to bend an 'approximate' vent line. I inserted this in the top gear tower hole and bent it to fit with several (many?) tries. Some bends could be made inside the tower, but most had to be outside and done on trial & error. I used this soft tube as the model for the real thing. Some minor bending of the real one still had to be done, but wasn't a problem. It does take some wiggle, swearing and manuvering to get the vent lines in, but they do go. Repeat the process for the other side as it will be a mirror image.
One other thought; as you get ready to mount the vent line, give consideration to the location of the throttle, mixture and prop cables and the fuel filter mount if you have one there - as well as any other things you may want to put in the tower (I have a push - pull cable to open and close the vent running in this area). By planning the vent line attachment screws, you'll avoid interference later.
Good luck,