For characters that don't already have N64-style models, I've found a way to still have them. And it's been in Brawlbox all along, even though I'm too ****ty at 3D modeling right now to help you guys as much as I'd like. Hiding the noobish terrible texture mapping and all that, here's ZSS:
it's simple. It's the 3D model for shadows, it's low-poly and on-point. If someone more experienced than myself were to texture this (I'm really, really out of my league trying to do anything 3D as my area of expertise is 2D right now), it would be the perfect model for her in this project.
The same goes for almost every other character that doesn't have an N64 model to correspond with.
Cheers!