This is a nice subject for a model boat, and its cool that its an older vintage hull! I'm also in the stages of gathering what I need to build up a kit that dates back to the same time frame as yours (in my case a 32nd Parallel XXIII).
My knowledge about RC subs pales to guys like Skip and Mike Dory, but I know I can at least answer a couple of your questions:
Current leveling devices for the tail planes are small and fully electronic. There is a Sub Tech unit called APC-4 (automatic pitch controller and I think Skip may have designed it..). A guy named Mike Caswell also sells one called an ADF (angle depth finder) which has a built in fail-safe circuit. These units cost between $70 to $80 and I plan to use one on my boat.
Freon has generally been replaced by canned air brush propellant. Although it was suggested to me, by Mike Dory, to use the canned air intended to be used for cleaning electronics, like "Super Duster". My kit has PVC pressure vessels for ballast and functional torpedoes. Mike said the air brush propellant has an added ingredient meant for lubricating the brushes that tends to soften the inside of the PVC, which can weaken it. The Super Duster air does not! He has a 50 year old RC U-Boat that still runs a PVC gas pressure vessel and he has had no problems with it (when you think about an RC sub that is half a century old...that is awesome, if that isn't legendary I don't know what is..). So it sure looks like I can use my stock gas cylinders instead of having to replace them with scratch built copper units. Bursting of the PVC cylinder should be a non issue do to the relatively low pressure of the canned air. Again Mike's sub, which is probably as old or older than most users of this forum is a testament to this. Mike, if your reading this it would be really cool to see a photo of the boat that I speak of here on the forum.
I've seen mast activation on here done by electric motors moving a mast platform up and down and by hydraulic pressure. My boat has a schnorkel in addition to a scope and I would like to try to get them moving possibly via a hydraulic set up. Believe it or not, I think there may even be some rather inexpensive Lego hydraulic systems that would work for this purpose. I need to investigate this area more myself, but a guy called Aqua Deep Dave on here, can probably provide more info on the Lego stuff. Ping pong balls or floats would be easier but don't allow you to raise and lower the masts at will when surfaced or submerged.
I like the photos you have posted and I will be watching your progress and comparing your notes with my own.