I would just get some apoxie sculpt.
The problem with sculpty is that it can slump when it gets too hot, or burn, or move, and it is also pretty brittle, and also you'll have to glue the clay again once it is baked- because once dry it won't necessarily stick to your pony as well. Also it seems heavier to me than apoxie.
Apoxie-sculp air dries and doesn't change shapes- it is sticky, but you can use gloves- and you can smooth it with water. Also it sands very well and dries VERY hard. And since it is essentially sculptable glue, you don't have to worry about it sticking once dry.