The problem with sandblasting a motor is that your blast media will get in all the wrong places. Think of all the little oil passageways. If you do not know what you are doing you can really mess it up -- and if you leave even one particle of media in there you can destroy a freshly rebuilt motor. It kind of puts it all in perspective.
I have blasted engine cases and had great success. I would not blast an assembled motor. When I have media blasted engine cases, cylinders, etc, I was pretty sure I knew what I was doing, I was careful, I planned ahead, and it all turned out OK. Some people will only blast motors with organic/soft (walnut) media. Another option is to use salt, which you can wash out with water.... Whatever you do, think real hard about it before you get started, plan ahead for what you are going to do about oil passage ways, bearing journals, case mating surfaces, to clean up afterward, what materials you are going to use to repaint the engine, and all that stuff.
As long as you take your time and work smart it can be done right.
Try locating a soda blasting company in your area. It's big with the car restoration folks. With soda they CAN just mask up the engine and blast it whole then rinse off the residual soda. It's getting quite popular.