Head swapping on smaller B&S?

Member mormorelawnmowers posted a question about swapping heads on a 12hp briggs and I have a similar question but didn't want to detract from his specific question.

My question is about the smaller vertical push mower briggs motors, flatheads. Which motors will the heads swap to/from if any? Just like the other guy, I want to bump compression. I'm wanting to try converting a pusher to propane and the higher compresions won't phase the propane a bit.