There have been some fan preservations, but the Godzilla fandom is much smaller and quite disorganized, so they aren't widespread and not many have gotten off the ground, plus finding good quality material for them is difficult enough. I'm hoping places like OT can change that.
It will be hard to find elements better than SP VHS tapes for the Titra versions of Sea Monster and Son of Godzilla, as those went directly to television in the late sixties. You'll be hard pressed to even find a reddened 16mm print.
Smog Monster on the other hand would be much easier. From what I've heard, the 1989 Orion video master was sourced from nice looking 35mm elements and there is an LD floating around, so that's one notch above the tape audio, plus the original AIP credits are in squished full frame 2.35:1, though I've heard the ending title card was cropped to 1.66:1 and then squished.
As for Destroy All Monsters, I have no clue where to find visual AIP elements outside of VHS quality bootlegs, but this video of the credits will give you an idea of what we're looking for: https://www.youtube.com/watch?v=oNt-5XCc1e0
There is an unauthorized DAM DVD on specialty Kaiju websites that has non-fluttery sound for the entirety of the film, so we may want to seek that out for the audio in favor of the MB release (Which I own).
For further information on the various English versions of Toho films, check out this thread: http://www.tohokingdom.com/forum/viewtopic.php?f=1&t=7462