As the others have stated, Kovu is no longer with FeralHeart as he left a long time ago; so there is no way to get into the coding because he only had access. c: Though, simple things can be edited such as markings, etc. Things that normal users can, essentially as Kerriki mentioned.
I wish things were different and I do wish he came back to at least share the code with the team so more can be done for the community. I would love to see that happen but alas, it most likely won't ever unfortunately. :c
I don't want to beat a dead horse, but the game's original creator has left. But we are fortunate that markings, maps, and the UV can be changed. And of course we have some really awesome modders in the community who've done well at enhancing the game, and I know some have even switched out the meshes for ears and tails with their own, but if you wanna spice up your game you can check here (https://feral-heart.com/smf/index.php?board=34.0) and see if anything looks cool to you ^^
In regards to the second question:
It's actually not that hard to make new models. Anyone with enough 3D modeling and coding experience can do it. I've done it myself with several feline ears and tails.
Like Jango_Fett said, the biggest problem is that you can't simply add things like new body part models into the game (as far as I know). You have to overwrite a preexisting model to add in a new one. If you want to, say, make a new tail, you need to choose one of the tails already in the game to replace. You also can't rename them in any way. If you make a feather tail to replace Ftail5, it's always going to be called "tailless" no matter what you do.
Still, given that it's possible to replace ears and tails, it should (in theory) be possible to replace the entire canine and feline models, but then you run into another problem: the difficulty of actually accomplishing such an enormous task. It's much harder to modify, convert, export, and properly import an entire model than it is to do the same with just ears. The process of importing and overwriting models involves finding very specific blocks of code in your original file and replacing them with new blocks from your exported file. The more complex a model is, the harder it is to accomplish this without messing up. If you don't know exactly what you're doing, it simply won't work. Heck, sometimes it doesn't work even if you do know exactly what you're doing. :(
(https://cdn.discordapp.com/attachments/555507327181127715/555534492329181215/screenshot_02152019_010408204.png)
^ this is an example of what happens when the code isn't 1000% perfect ^
ALSO, ofc, what we can replace is limited by what we can access. While it's possible to replace the bodies, ears, tails, and manes, I don't believe it's possible for the average person to replace the wings. There isn't a publicly available .blend file that includes the wings and their skeletons. Without those, the best you'd be able to get is an item.