The key between pulley and shaft is meant to be a form of mechanical fuse so it breaks if the torque rquired to turn whatever is being driven exceeds safe limits.
Two possibilities here, one the key was old and weak and just gave up the ghost. Two is that they didn't replace it, or if they did, it failed prematurely for some reason.
Either way, you need to get them to look at what caused the key to fail. If they set the crank 90 degree BTDC so the pistons are halfway down the bores, can the camshafts be turned easily? I think (although not 100% sure) there is an oil pressure restrictor valve in the top of the block. If this has failed, it will starve the camshaft bearings of oil causing siezure. If this has happened it's a head off job.
Think you said it was a replacement engine from somewhere? Was the oil and filter changed when it was fitted? Negelected oil and filter changes can cause those oil restrictor valves to block/fail.
Needs these basics checking out before they start replacing the keys and pulleys and whatever else and potentially turning the valves into bananas.
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Cheers
Dave
Next Door to Top-Gun with a Honda CR-V & S Type Jag  Volvo gone but not forgotten........
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