Hello Steve! I have just completed what I believe to be the first of its kind. I have built your SP5 clock at 50% Scale! This was a fun idea I had and I wanted to take on the challenge to see if this was possible. There were several hurdles along the way, but I finally figured it out. Listed below is how I completed this project. Since the gears were much harder to print cleanly, I printed this clock with the 7 day runtime to reduce as much friction as possible, and to use as little weight as possible.
I was able to find screws in almost exactly 50% size as well. I used M1.7 X 8mm flat head self tapping screws. I had to drill out every hole that needed a screw with a bit just barely smaller than the screw since I broke a few screw heads off without drilling first.
Another challenge was the weight shell. When I scaled the largest diameter to 50% I couldn't fit all my lead fishing sinkers inside of them. It was also hitting the wall since the clock is much closer to the wall. What I found was that I scaled the 2.4 Inch diameter weight shell to 85% scale and it is almost perfect. I was able to use the original screws from the SP5 for the weight shell even at 85% scale, I just needed to drill out the holes for the bottom cap. All of my weights fit inside now, but it barely skims the wall, so I may need to make the clock frame back top and bottom a little thicker to bump it out from the wall a bit. I also did not use the pulley system. Using the pully system would need somewhere around 4-5 pounds for this clock. I didn't want to break the frame from too much weight since the screws and printed parts are weaker than the original. With the string coming straight off the winding drum I can run this clock with only 2 pounds of weight, which I am happy about.
The last challenge was regulation. The clock always ran fast no matter what. I wasn't able to add any weights to the pendulum bob due to the small size, so maybe that is the reason. My solution was to just keep adding pendulum section lengths until I could use the pendulum bob to fine-tune the timekeeping. The final pendulum is around 19" long now! It looks a little unproportional, but I think it's funny to have such a small clock and a large pendulum.