Well, I got the heads off. Surprisingly/suspiciously easy. Videos of heads and cylinder walls are below. Rust stains on head gaskets. No gouges or horizontal marks on walls. There was what felt like a hard rubber tube in the gaps above and below the center cylinder. I don’t know what those are or if they’re functioning.
My newbie guess is loose or warped heads.
It may have overheated when it was still in use by previous owner(s). One overheat sensor was disconnected when I got it. The other came apart—wire pulled loose— while I was trying to take it out to remove the head.