In the parts diagram, the spring, Item 7 has a 90 degree end on the inside that goes into a hole in the crankcase to keep the spring from turning. The other end has the end bent at 90 degrees pointed radially inward toward the center. This end goes into the hole drilled through the shaft near the outer end. The spring is wound ccw about one turn and the outer end inserted into the hole in the shaft. Then it's tension twists the shaft cw against the internal stop (inside the transmission), bringing the kickstart lever back up.
Unfortunately, I missed a possibility; if the shaft was assembled into the internal ratchet holder, Item 3 below, with the wrong orientation, the hole in the shaft will have the wrong orientation and it may not be possible to tension the spring properly. The hole in the shaft should be vertical whent he shaft is cw against the internal stop. See the 50-175 Service Manual page 21 for a photo. If the orientation is wrong, the crankcases may have to be split to correct it. Hopefully, the orientation on yours is correct and the assembler just didn't wind up enough tension on the spring.