You have to borrow articles from other characters and modify them. For the most part, it's possible to make them look completely new with some originality. it requires more than just changing stuff in PSA because assembly coding is needed to clone those articles (as well as the characters). Furthermore, it takes a lot more work than what you'd normally find from Brawl Vault builds to make a Project M quality build. I don't mean to say the work done on Brawl Vault is lazy or terrible, but we have a different take on what makes a quality build. It has to work within the confines of Project M's engine, follow certain standards, etc... In some cases, one could also potentially use some rigging magic to make a projectile look different for example.
So yes, despite the fact that we have limitations to contend with, we can do some amazing things within those limits. The work load necessary for those things is pretty intense however, so it has to be worth it for us to consider a large article change.