I don't have the horse Indy, I have seen it at the store and I do like it, it's next on my list. But you should be able to just pop the torso off the legs, I can do that with my pitfall Indy but when I tried before on my ROTLA Indy it was harder. But I'm gonna try again. The arms will look fine, I have obsereved alot of the Indy figures and have noticeed alll the ones with jackets have the same upper arm sculpt so all you need to do is pop the forearms out and replace them. Good luck
edit: I recomend when popping the torso off the legs remove the head as your hand might pop it off the ball joint and it could get lost (happened to me with a star wars figure) and romove the jacket too, it will make it easyer. Then just grab the legs with one hand, grab the torso with the other and just pull, pull upwords though becasue if you pull sideways it could snap the peg the torso is mounted on, that has happed to me with SW ones and with 12 inch figure heads.