JD 425 head gasket question

My girlfriend was mowing with John Deere 425 yesterday and texted me to say it started running real bad. Then she said the temperature gauge was high. When I asked her if it was in the red she said it may have been in the red but not for long... of course, I don't know if she looked at the gauge before it started running bad. She said she shut off the mower right away then drove a short distance to garage and shut it down completely. When I got there, I noticed the anti-freeze was way down. I didn't notice it leaking anti-freeze or any other leaks. I added quite a bit of anti-freeze then parked it in the garage. When I was backing it in, it started running real rough like one cylinder wasn't working. Today, I took off the side panels, and when I took off the side that was running real bad, something fell onto the ground. It was a spark plug. The center of the spark plug was loose, I could slide the porcelain center up and down. I got a pair of new plugs and started it up. It seemed to run faster than it should and doesn't sound right. I didn't run it long enough to see if the anti-freeze went down. I have a compression tester. If I test the compression what should it show on each cylinder if one of the head gaskets is bad? Does this sound like a blown head gasket? I didn't notice any excessive amounts of white or blue smoke, nor did I smell anything weird. But as I said, I didn't run it very long. Any thoughts would be appreciated. If its a blown head gasket is that a pretty straightforward job on this garden tractor? Should I do both at the same time?

I should add the oil level was right up to the full mark and looked like oil not bubbly or full of anti-freeze as far as I could tell.

Good afternoon, Dromunds; If the spark plug fell out, that is one problem. If the center part of the plug is loose, that is another problem. Neither of these should be related to a head gasket problem. My thinking would be to plan on putting in a new spark plug, but first check compression. As I learned in army electronic school, everything you touch is one more thing you need to be suspicious of. Good luck!
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Dennis M. in W. Tenn.
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My 345 had a head go bad - the actual head, not the gasket. Symptoms were similar, but much more severe than you describe. I could not restart the tractor though. And lots of white smoke before it died.

Tim

If the engine is a Kawasaki FD620D, (according to jdparts.com), the Kawasaki service manual states 170 psi minimum for compression.