Here's a detailed list of everything I know about the engine trouble, and everything I've tried (I think).
Engine trouble
Developed very suddenly ca. 13 December 2012.
Running very rough, such that engine is shaking hard against its mounts. Revs will not rise above 1700 and usually not above 500 without lots of persuasion. Depressing or releasing accelerator does not vary engine speed by any appreciable amount, nor does choking or increasing airflow (though holding accelerator down for long periods does very gradually increase engine speed). When engaging clutch, engine slows; when releasing clutch after engaging a gear, engine invariably dies, though it seems to just barely like reverse gear more than the forward gears.
Engine is smooth and easy to turn over by hand; with plugs in, compression stroke seems strong; with plugs out, turning is smooth and without hang-ups throughout revolutions. When turned by hand, engine makes no strange noises. Concluded from this that timing chain has not snapped or jumped a tooth.
Carburetor damper was sticking. Damper cleaned, needle and jet replaced, damper moves easily now; no change in engine running. Accelerator and choke cables readjusted; no change. Having air filter on or off makes no difference. Carburetor float does not stick and seems to be functioning correctly.
Fuel pump delivers plenty of fuel and fuel filter is new and unclogged. Tank has enough fuel in it (about ¼ tank at this writing).
Valve gaps were reset prior to trouble developing.
Static timing and points gap was reset when engine was pulled, ca. 20 December, and made no change. Turning distributor either way (advancing or retarding timing) makes very little change. Changed condenser 9 January 2013; no change (but replacement condenser condition was unknown). Distributor has not been removed for at least two years, and running was good prior to 13 December, so timing is not 180 deg out. Cap is in good shape and shows no tracking or cracks. Points had some slight burning/oiling, but cleaning and filing them made no difference. Resetting points gap to 15 thou made no difference. Changed points for a brand-new set 11 January, set to 15 thou; no difference.
Oil changed 8 January, after refitting engine. No change.
Sump removed 1 January for cleaning and inspection of engine innards. Sump had very little sludge and no metal shards. Crankshaft appeared in good condition with no cracks. Camshaft appeared in good condition as well, as did distributor drive.
Clutch pressure plate changed 5 January for an unrelated problem; prior to this, flywheel and engine backplate were removed for renewal of gasket. Oil pump was like new. Flywheel was refitted correctly and clutch was reassembled correctly.
Battery was recharged prior to trouble developing, and since, and this made no difference. Starter functions well with lots of power.
There are no indications of head gasket trouble.
Plug wires changed 10 January; no difference. Each wire sparks against its plug, though Cylinder 3 seems to be receiving a larger spark than the other cylinders.
Another compression test conducted 10 January. Compression better than before! Cylinder 1: 130 psi; Cyl 2: 140; Cyl 3: 140; Cyl 4: 140.
Plug 3 ceramic casing cracked 11 January; changed for new; no difference.
Still waiting on taking it to a mechanic...nothing gets done when I'm not there to do it.
