Actually now that i'm thinking about it would it have been possible to retexture them or put new models on top of the old rigs?
People like these models because of how animated they are so surely they could have just edited the model on top like they did by editing Hau's model to make Hop.
They'd probably have to remake the models from the ground up regardless considering N64 models are uh... very outdated and they won't be able to mask it up with high quality textures when the poly count is so low and the gake is HD.
And if they have to remake the models from the ground up, then there a very good chance they have to remake the animations because sometimes the rigging would be off and they gotta change that. Well, of course they don't have too, Tekken is king of reusing animations, and that's fine and all but let's be real some of them look super jank nowadays especially for old characters when you compare them to the new characters. With Pokemon, that's over 500 characters reusing and recycling old animations for game that's no where near as frantic high paced as Tekken, there's no way they'd get away with it because a lot of mons would look janky as hell. As for the Hau and Hop thing, they future proofed that so the animations are less jankier and much better to use.
TL;DR they are gonna have to remake the models regardless because of the low polycount (and because some mons would look ugly and out of place), and it's for the best they remake the animations because it's not gonna blend well with the new mons.