The welded loop on the "front" end of most fly lines is a relatively recent innovation. On lines lacking such loops I've always just added a short length (8-10 inch) of 20-30 pound monofilament, nail-knotted to the end of the line, with a perfection loop on the other end for a loop-to-loop connection to the the leader.
If you want to get a bit more fancy, a needle nail-knot in which the monofilament is inserted into the core, brought out through the side of the fly line's coating about an inch from the tip and nail-knotted there looks a bit neater and, since the monofilament comes directly out of the center of the fly line, seems to pass though the tip-top a bit more smoothly.