Those external oilers are usually made from 1/8" OD tubing, the same size as the line going to the dash gauge. The inside diameter of the tube is only about 1/16", and it gets clogged easily. The tubing is available at most FLAPS in lengths sufficient to go from the engine to the dash, with an assortment of fittings. I'd suggest replacing both lines, the one from the engine to the dash gauge, and the one to the head, if it's necessary to get oil to the rocker arms. It's not uncommon for the internal oil passages to get plugged up on an old, high-mileage engine.
Jerry
