Cleaning the water jacket area

PS-added benefit of the washing soda-your wife can then use the rest of it to wash your greasy clothes, she'll like the results.

The inside of the engine does not look too bad. Not much point in worrying about that because the only rust you will be able to remove is what you see, and that is only a very small part of what is actually in there. The ONLY way to get rid of all that rust would be to strip the engine completely, remove it from the tractor, and take the block somewhere and have it chemically cleaned.

You need to key on the words "the inside of the engine does not look too bad".

Also, when you go to take this cover off again, it will not have been on for twenty years, and will come right off with no twisted bolts.

It would be a good idea to get some anti-sieze compound, such as never-sieze, to lubricate your bolts with before reassembling. Bolts should always be lubricated. When you assemble dry threads you run the risk of the dry threads resisting, making you think the bolt is tight, when it is not. Well lubricated threads always make up the best and are less likely to gall and pull a thread upon assembly. For instance, I always squirt oil on lug nut threads before putting a wheel back on, whether it is a tractor, truck, or car. Not only do they come off easier later, they are more prone to stay tight. This is counterintuitive to some people, but it is very true.

The anti-sieze compounds are good to keep water out of threads which are exposed on the water jacket, like the bolts holding the thermostat, and are good for high temperature like the manifold nuts. It even helps on the muffler clamp.