Why didn't I get Adobe Lightroom earlier? Even its basic photo-editing tools dramatically improve photos (or at least get them back to where they should be). I'm going to re-scan this one at higher resolution, after carefully dusting it, and with the appropriate filter, but for the moment, I think I've gotten pretty close to what the original Kodachrome image looked like:
This one came close, but not quite:
Both: Public Garden, Boston, 10 May 1986. Kodachrome 64. Exposure unrecorded.