How to clean the engine inside?
Yep - machine shops around the world put the block either in a "hot tank" where the block is put in a bath of caustic soda, or in a spray booth, where a bunch of jets of the same liquid are sprayed at the block. Either way, it removes rust, scale, just about anything. Any machine shop that rebuilds iron block engines should be able to help you out, no matter what the brand.
Be sure that you put oil in the cylinders after you get it back, as the iron will start rusting immediately.