Some untruths in this thread.
Let me share some truths.
N13 exa has 2 belt crank pulley that fits on s13 ca18.
S13 silvia pulley is 3 belt, and one pulley can be machined off.
You cannot make a 2 or 3 belt pulley into a single belt pulley without compromising the rubber inside the pulley.
None of the ca18 pulleys are solid, all are rubber isolated. This sounds like marketing hype from Ross sales guy.
The ca20 pulley looks the same externally as a ca18 one but has smaller crankhole and is not interchangeable.