Oh well ....
I drove it back down without much force. A socket and a few gentle hammer taps. With not many other economical (OK cheap) options, I decided to goop around it with JB Weld metal mender and let it set up for a couple of days. Went back at it this morning and put it together. The spring, retainer and keepers weren't as bad as I was expecting. Set the gap/clearance on all four valves. Closed it all up and turned the key.
Surprising it started fairly easily considering it had been setting open to the air for a week or more. Idles better and runs smoother. Ran it around the yard a bit, but didn't tackle the hills. It shut off without spitting and coughing like it had been, then started back up again without trouble and shut off smooth again.
Jury is still out. I'm not sure if I expect the JB Weld to hold or not, but if it doesn't, I may be slightly more inclined to try a head if I can find one for less than a small fortune.