Chains break I've seen it, replacing them as maintenance is a good call.

Before removing the pump, clean all the fuel connections and scribe the pump adjustment/rotational position as it relates to the block. Zero the crank and camshaft, pull the pump, line up the index mark on the front of the pump, lock the pump, reinstall the pump, remove the locking tool, align the scribe marks, bleed the fuel system and it should start.

To dial it in I'd suggest drip timing the pump or having it electronically set/timed.

Good luck!!!

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