I assume this is pretty normal (almost mirror polish on the cylinder, doesn't come across well on, ahem, 'endophotos') and I take the staining on the piston as just surface tarnish/a pretty confirmation that the Italians can sure design a combustion chamber flame path old Dr. Schnuerle would approve of.
Checked AS, checked compression again.
Looked at rings through exhaust port, look fine. Piston ditto.
Reckon it'll run.
Reassembled ignition module, plug in lead held on cylinder, no spark.
Checked plug, resistor plug, no obvious defect. Replaced with new (also resistor), no spark.
Checked AS, checked coil. 2.234kOhm secondary, 153Ohm primary. A little low, but in the ballpark. No short.
2V on primary while pulling reps. Barely 1V secondary. Boo.
So module busted (old caps? blown tranny? who knows). Quickly discovered low to zero chance of finding good replacement module. Boo.
It gets good now.
Took inspiration from earlier in this thread where Ed suggested Jerry could rehome a 2-pole Selettra coil onto a 3 pole core like mine. Had some doubts about trigger circuit compatibility and timing with that specific solution.
Discovered Selettra make a current 3-pole magneto ignition module that's conceptually very similar, the I27A, for karts. Italianly expensive though (liek, 400 bucks omigod).
Discovered eBay is awash with chinese knockoffs of said module (search "ignition coil pit bike" or so). Can't find dimensions of the 'real' I27A, but the 264 has 67mm between bolt holes, and the coil itsself is 45mm across (and easily removed by snapping the retaining clip off the secondary blister and applying elbow grease and a light drifting).
For $12 I decided to take the further punt and ordered a "52mm IGNITION COIL 33CC 36CC 39CC 52cc Chopper Mini moto GAS Scooter GoKart Bike", not even from China but from one of the local container distributors for because impatience.
It arrived yesterday. Secondary resistance: 9.75kOhm. Primary resistance: futile. Well I might as well try...
Had to cut the edges off the resin/plastic where it was potted to the laminates.
Had to use substantially more drifting to remove it.
Comparison.
Oh the miracle of zipties (note flywheel clearance is yes).
Got out my remove-plugs-from-the-possible-unknowns. Although this tool looks custom made specifically for checking for spark on an SAE-compatible ignition without risk of electric shock, it is actually 3 things I found in my dad's Orange Tub of Spare Things when I was 5 or 6 years old and pliersed together (p.s. I'm 40). Worked though.
Weak little sparklets they were, but they were there.
Reckon it'll run.
Now to glue the new coil to the laminates...