Hello,
the K 1000 is in principle a Spotmatic F with bayonet and the SP F is nearly identical inside with the Spotmatic. So if you'll find a repair manual for any of these models, it will help you with the K 1000 too. Afaik you may have to remove the galvanometer and the lightmeter/film speed potentiometer before you can remove the prism which is secured by 4 screws and 2 springs. The focussing screen can be adjusted from inside the mirror box by screws (3 ?) and could be changed by a Pentax service against a split image type.