Have gotten the engine torn down, and in addition to the known weak valve springs found some light scoring in one cylinder wall, a little pitting in another, and some scoring on the crankshaft and bearings in a couple places. Tentative plan is to bore the cylinders to 82.5mm for new pistons, regrind crankshaft, replaces valve springs, and skim the head - but first will have it pressure tested and examined to make sure there aren't any hidden issues or reasons a boring isn't possible.
I think I mentioned in an early post in this thread that a letter in my files written by the 2nd and longest owner (30+ years from the late 60s) indicated he'd thrown a rod at Sandown in his only time on a track and rebuilt the engine after having the block repaired. On teardown, I found that was #3, based on block wall welds and corresponding damage to the sump. Looked pretty durn violent! Glad I have a backup rope seal sump, as the original has been well abused!