I don't know of a way to make a USD fork longer. As for going shorter, I'd rather get real close to the target ... meaning, I wouldn't want to get a CRF450R fork if I only wanted 4.5-5" of travel.
I've never done a Moto Guzzi fork swap, but I'm done several on dirt bikes. Many challenges with a project like this.
First one is space up top. Most times with a conventional fork bike, the designers/engineers take advantage of the smaller tubes up top. Tank location & width, steering stops, frame. Switching to an USD fork means significantly thicker top tubes and the triple clamp that goes along with it.
Then for the front wheel and brakes ... you have axle diameter, and spacers to think of ... and then where & how the brake caliper attaches. Also how wide the hub in on your bike vs. the donor bike.
I've found the easiest thing to do get the entire front end for a donor/target bike: triple clamp, front brake (master & calipers), forks, front wheel, spacers ... it makes your life WAY easier.
If you want a wire spokes front wheel, I'd look at an Apriila Caponord model that had those.
A low cost first step is to buy the triple clamp of the target donor bike first ... then you can check out clearances on the frame and tank ... and the length of the steering tube, bearing sizes, etc.
If you look at this one ... looks like the turning radius would be pretty large, meaning those top fork tubes don't have a lot of room with that tank. Also, not sure I like the front end high stance it has.
Where this one, designed as a scrambler, seems to have plenty of tank/fork room:short sorry poems