Engine died!

My B18 which I rebuilt in 2009, about 50K miles ago, just died when exiting the freeway. It's the hottest day of the year, 104 degrees in the shade, about a million degrees in the sun. The car hummed happily along at 70mph, as usual, not breaking a sweat, coolant temp gauge perfectly centered.

When getting off the gas from 4,000 rpm to go down the freeway off ramp, I heard a clong, followed by a few more clongs, then silence. At the end of the off ramp, the engine was off and wouldn't start again.
When activating the starter, the engine turns quickly, making a sound like a well-balanced coffee grinder. The radiator fan is turning.
I removed the valve cover, and none of the valves are moving. My guess is therefore that the Bakelite timing wheel failed, and parts of it fell "down."

My questions are:

1)can I replace the timing gear by just removing radiator and water pump, but without pulling the engine?

2)should I go with steel instead?

3)am I correct to assume that there's no way a piston could have hit one of the valves?

Thanks, guys!

Bernard
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My Volvo, so far on 8 pages at: http://www.californiaclassix.com/Bernard/PV544-1.html