How does the engine run with the fuel pump and the pushrod removed, with gas supplied by an electric pump? What is the stroke of the fuel pump pushrod, measured with a dial indicator? A disassembly would be a last resort if the engine came to my shop with that noise, and only after some detailed diagnosis. I'd check the flywheel bolts for the proper torque as one of the first tests.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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