It's not the lens, because the lens is on the other side of the shutter from the film. Any light leaking into the mirror box, whether from the lens or anywhere else on the front of the camera, will only illuminate the film for the (typically) split-second the shutter is open for each exposure.
Light leaks don't have to let much light in, because they leak continuously anytime the camera is exposed to light. Remember that the top right of the frame is on the lower left (as viewed from behind the camera) when the negative is inside the camera. So look at the seals around the back door on the lower left.
You can pick up 3 sets of pre-cut new seals for the K1000 for around $25 on eBAY.
Instructions for how to remove the old seals and install the new ones can be found here