A Schneider G-Claron is a great older lens for macro work. The only problem is you'll need to have the Photo Fairy transform your Pentax into an 8x10.
Seriously, I'm not sure what is available for the Pentax for macro work. Aside from the lens, you'll need the capability of much greater lens extension for the close focusing - either a bellows unit or extension rings. Often, reversing the lens via a lens-reversing adapter is needed to get optimal sharpness from a conventional lens at macro distances. True macro lenses are corrected for the close focusing distances, whereas conventional lenses are corrected for infinity focus.
Longer focal lengths require even more extension, but provide the convenience of somewhat greater lens-to-subject distance, making lighting more convenient. Then, there's the problem of DOF being very limited at macro distances. So, a lens with smaller apertures may be a better choice.
Lots of trade-offs in the macro realm, and there are a variety of solutions based on what you want to achieve.