IIRC, it's an oddball diameter, maybe a 7/16" OD? I believe you can change the compression fittings in the oil pump and the block to 1/2" OD, and use either copper or steel tubing and a tube bender to fabricate one. Be careful to allow for clearance for the crankshaft.
Jerry

Edit: If you're talking about the small-diameter tube that goes through the water jacket to the lifter galley, several of the online antique parts vendors sell a kit with new tubing and all the necessary fittings to replace it.

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