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I thought of ROD for a number of reasons. (Actually, rod end! but, hey; it's still a rod part.
The engine was idling somewhat fast.
It had been "recently" rebuilt.
We weren't sure about the oil, and it was on the sidestand, suggesting the right side might have been starved.
I thought of a valve going tight early this morning, too, but am just now getting on line.
SO...if cin STOPPED the engine and a valve guide dropped or something like that...bent stem, seized stem, whatever, it might not be as bad as it could be.
Pulling the valve covers off to have a look there is a good first thing to do.
Since it's Thursday afternoon now, we ought to get some word soon about what's been found.
The "...gasp of air and then the metallic bonk..." suggests a valve interference more strongly than a rod.
The pistons are right stout, too. Hitting a valve when you are rolling the bike MAY not have hurt anything. Only inspection can tell.
BTW, cin. I didn't notice any model/year mentioned in your earlier posts. Tell us a bit more so if you need parts someone can help.

Thanks, Jean. It's a 78 R100/7...I haven't made it back home yet to inspect further. I will investigate this weekend and update with whatever I'm able to determine. BTW, I work in the Seneca, Clemson area a couple times a year and have seen you guys riding in late January...lucky bastards.

Any day when the temps are above 40F and there's sunshine is a day to ride. Once I rode to work when it was 30F...the guy on the radio said 40, and I didn't think to check the thermometer on the porch. About half way to work, I felt like I'd been hit on the back of my head with a bat!! (Really cold does that!) Only 2 bikes in the parking lot that day. Other one had full faring and wires for heated garmets.
I think I rode this year in January and February, too. Gotta keep those iron rotors clean.
Send PM when you wil be down this way. We can have a beer or 2. or 3. or 4.