And that stuff in there might not be rust. Congealed castor oil looks a lot like rust or varnish. Soak the engine in some fuel and if it is castor oil, the fuel will dissolve it and you can flush it out. Worst case, if it does turn out to be rust, you have to remove the cransshaft to replace the bearings anyway. At that point, it is easy to clean the rust off. Clean that sucker up and replace the bearings andprestoyou have a new engine. Remember, the only things in the engine that can rust are the bearings and the crankshaft. You have cleaned the crankshaft and replaced the bearings, so no more rust problem.