Home Improvement Stores, Hardware Stores, etc sell a Fibreglass mesh tape for taping plaster cracks, imbed that in the plaster and your breakage problem will be reduced. It is cheap, and works well. You can also glue it to the back of already cast parts with Elmer's glue and it will help to strengthen the parts.
Also works great for making fences, windowgrates etc., lots of uses so your money won't be wasted.
George