The Pentax K.1000 Am-meter has slightly larger dimensions but will fit in place of the SP am-meter by making a small modification.
First cut off the red wire as you will only need the Yellow and Blue wires.

Because of space converns you must remove the plastic insulator cup from the bottom of the K1000 meter and tape a piece of insulating tape around it as shown.
Next use a sharp knife to trim away the lips each side (where the two little hands point to). This will reduce the width of the meter sufficiently for it to fit in place of the old meter.

When fixing the meter in place, ensure the needle is positioned at the lower edge of the centering frame in the finder. This is to ensure accuracy and allow for an OFF position for the customer.
Also note that there may be a slight decrease in meter sensitivity. This is of no consequence.

The old CDS cells can be replaced by a single cell. We have succesfully been using an Olympus OM.1 cell, but any other cell with similar charateristics can be used. If using the OM1 cell cut off the PINK wire.
First scrape off the black paint from the top rear of the pentaprism as shown, and stick the CDS cell on with a little dab of superglue (gel type). Then seal all the way around the cell with black tape. The metering now becomes SPOT metering with greatest sensitivity in the centre. Because this makes it more sensitive to stray light from the eyepiece, seals the corners of the prism with tape and tell the customer to have eye close to finder especially under low light.

Here is the wiring using the K.1000 meter and the OM.1 CDS cell (or the original cells).
NOTE: Because the OM cell best matches the BLUE Pentax cells, you may need to make a correction when substituting for the RED or WHITE Pentax cells. Fit a 20K pot in line with the yellow ISO wire and adjust, then replace the pot with a fixed resistor.