I got a Newgrounds account just so I could comment on this post!
It's not every day you find someone who remembers such an obscure character and I was super happy to find that out when I first discovered this post last year! And now I can finally say that
I have the model!!!
A few months ago, I was able to find someone who was able to rip Billy's model from the game! but it took quite a few months for the model to finally be finalized how it is now since the person who ripped it for me went silent shortly after saying they were talking with someone overseas to help with figuring out how to apply the textures
Months later, I finally found someone who was able to help me restore the textures! So now I have a finished model of him! the model also does have an armature that I made myself, but the model isn't rigged since I haven't properly figured that out yet.
If you would like the model, I'll happily hand you the .blend file if I can!