Wow, that thing is rusty! I wouldn't really worry about the rust on the chassis per se, although you should clean it up, but the rust on the transformer would be more of a concern. When an output transformer rusts liek that, it rusts internally too, and the plates inside short together, causing it to fail. If it hasn't failed yet, it will.
The stuff I was talking about is made by the same people as POR 15, but its a rust remover thing, I can't recall the name offhand. POR 15 is a black, sticky substance that you put on metal to protect it once the rust is gone, like on a car restoration or something like that. All you need is the rust remover stuff, whatever it is called. You just spray it on, then wait a bit, and then wipe it off, and the rust is now a dark brownish, gray color and you can sand it smooth. You could use it inside the chassis, but you would have to take all the parts out before you did so as it would ruin most of them because of its acidic content.
What I would probably do, would be to get a large sheet of paper, and draw out the layout and label all the parts. Like R1, R1, C1, C2, etc. Draw out all the wiring connections and colors of wire, and then you can use that, and your pics to put it back together after you have cleaned it up. Then go ahead and take everything out, and take the faceplate off the front, so you have just the bare chassis. Then get the rust removal stuff and spray it on, wipe off, sand, spray on, wipe off, sand, etc, until the chassis looks like all the rust is gone. And try not to get the stuff in the tube sockets, or you'll have to replace them too. Then you can spray some clear rust-olium over the whole chassis so it won't rust again. Then put it all back together again. You would probably want to get new transformers too, although the rust on the power transformer doesn't look nearly as bad as what is on the output transformer. When you have the transformers disconnected from the circuit, if you have a signal generator, or access to one, and a DMM, you can figure out if the transformers are shorted or not. If you need to replace one, you can get an exact replacement from Triode Electronics.
Doing all of this is pretty much the same as building an amp. It can be done, and if you do it all correctly, it will make a great sounding amp, and you will learn a lot in the process. I did this to a 1970 Sonic 1 that I got off ebay. It is pretty much the same as a 200S, so I made it into a 200S since the labelling was all worn off. It works and sounds great, after a complete rebuild by me. Mine wasn't as rusty as yours, and was complete, but didn't work right. After replacing every single ressitor and cap in the amp, and making some tasteful mods, it works really well and I learned a lot too.
You'll find if you decide to go this route and rebuild the whole thing that modern resistors won't fit in the center section above the 6an8 tube because the leads are too short, unless you use 2 watt metal film resistors. The resistors that Ted Weber at webervst sells have longer leads than most, so his 1W resistors might fit. You could also try to find some NOS carbon comp resistors like what was originally used in there, but personally, I think the metal film is a better choice for noise reasons, and it would sound better for a bass amp too. For caps, use whatever you like...most of the modern stuff is light years better than the older caps. I used Sprague 715P polypropylene and Sprague 225P polyester, ceramic, and Xicon electrolytics depending on where in the circuit I was using them. I woudl recommend to not bother with the can cap, and use discrete caps as they are cheaper and will give a better voltage rating as long as you hook them up right. I can provide more details on that and on my Sunn, and some pics if you email me. Good luck!
