Pressure leak-down testing without the regulator/gauge set, and air compressor:
Pull all of the spark plugs from 2,3,&4. Leave #1 spark plug in. Disable the ignition. Cranking over listen for the compression leakage with a mechanics stethoscope at either out the oil filler - THE PISTON RINGS.
Or: hissing pressure leakage out either the intake entry or exhaust manifold - THE VALVES SEATS.
Repeat this test doing cylinder #4.
The rings on a low-use one like this can always be in-place decarbonized improved with a patent spray-into-the cold cylinder de-carbonizer. Foaming Sea-Foam brand; a foaming spray-in Chrysler product, ect. Left overnight to sit and work.
Not-sealing valve(s) at the seats is a pull the head, and repair job.
Yep. Yep. You will have to put back in the push rods and rocker arms to do this test.
I am actually surprized this 1-3-4-2 engine will run on just two cylinders with usable power. 3&2 still good and I think you got lucky. 1&3; 3&4; 4&2, 2&1;;and I do not think it would.