I've nearly finished fitting a 20mm PX disc fork into a VLB (sprint) and I can tell you it is not as easy as you would think (and I'm an engineer!).
The point about the front shock is correct. A standard shock will make your front end sit up by about 35mm giving your scoot the dragster look. You have two choices, either raise the rear damper to match with a longer damper stud at the top where it meets the frame (you will need a longer stand or side stand) or fit a shorter shock on the fork.
The fork profiles where the fender sits are different. The mudguard (fender) will need the hole where the forks fit reshaping to match the profile of the top of the new forks. And new holes will have to be drilled.
The PX/ PK forks are wider than the original VBB fender (where the shock sits) This will take very carefull measuring and marking in order to work out where to either reshape or cut. I have cut a hole in mine and welded in a formed section to match the bulge where the shock sits.
Even when the mudguard is sitting level and square with the centre of the wheel you may find that when you come to fit in the frame the forks may sit too far up the frame and the fender rubs on the bottom of the horncast. You may need to trim this too.