The Bowser Moveset API/template is surprisingly easy to use, I put the shell spin on my local copy of Dry Bones in like ten minutes, would have been like a minute if I knew what I was doing.
Unfortunately, I don't know how to get a hold of and recolor the regular Koopa Shell model, so he grows Bowser Jr.'s shell every time.
Also the playable regular Koopa Troopa only seems to be on Gamebanana?