Starting problem

[Tom in France]

A new member posting for the first time. I inherited my father-in-law's 1988 Volvo 240 sedan around 15 years ago. Its 210,000 kilometers have been relatively trouble-free. I replaced the timing belt, water pump and all of the drive belts a year ago at around 208,000 kms (the vehicle is rarely used).
This summer, the top connection on the radiator failed in a spectacular, crumbling mess, as the bakelite plastic disintegrated under the pressure of the hose clamp. I only realized that this had happened because I had stopped to ask a man working in his front yard for directions. He told me that my car had water running out of the motor. Just as I touched the key to turn it off, the motor shut off by itself. Since then, it has refused to start.
I have checked the fuses, changed the fuel relay, and I've changed the main fuel pump and filter. I know that it is getting fuel, as I have disconnected the return fuel line at the injection rail. When I crank the motor, I get fuel out of the line.
I checked the coil, and I have 1.2 Ohms primary resistance and 10,6k Ohms secondary resistance. There is what appears to be a weak spark when I remove a spark plug and ground it against the motor.
I checked the compression in the cylinders. One of the cylinders had only 75lbs. with the three others ranging from 95lbs to 120lbs. I added a teaspoon of motor oil to the cylinder with the lowest compression, and it came up to 100lbs.
I would appreciate any suggestions or advice that the forum members might be able to offer. I hope that I didn't blow the head gasket, but I didn't think to look at the temperature gauge when this happened, and I don't know how to check a blown head gasket, other than looking for foam on the engine oil cap (there is none). When the motor shut off, it was like there had been an electrical problem, as I didn't hear any signs of engine seizure or a mechanical malfunction.
I might also mention that I also checked, when the oil cap was off, and the crank is turning. So, I take that for a sign that the timing belt is still intact.
Thanks in advance for your suggestions.