This question: How do I use the "Simics Training" and "QSP CPU" packages? answered how to change the CPU. But I tried the same-ish steps of searching by strings to try and find an alternative to the default northbridge/southbridge, but I didn't see other options for those. Nor did I see any way to change to a PCH-based system.
Is there a way to change to a PCH-based simulation in Simics 2021.24?
Not currently. Only the QSP platform model is currently part of the public release of Simics.