Rebore done, bores were worn very oval, but side-to-side, not front-to-back. I really don't understand this, as there was very little piston wear.
All I can think of is that the cylinders had distorted at some stage, maybe due to lack of coolant?
Front was much worse than rear. A strange problem, I hadn't found this before on these engines.
Or it's been apart before?
That explains the lack of compression, anyway.
Ring gaps were larger than max specified wear.
Either way, the rebuild's now under way, +.5 pistons (the only oversize available from Honda) with their new rings are in place, settled in their bores, crankcases back in one piece, and all properly torqued up.
New mains and big ends fitted, and oil pump back in, checked and ok.
A few pics:

It's not falling off, it's an upgrade opportunity.