You can go to the SPA site and read the rules.
Any design prior to Jan 1 1976 is legal. This includes the Curare but the Tiporare is not on the list as it came later....however the Utter Chaos and Killer Chaos came much later and are allowed...so there is some "allowances" in the rules.
Engine must be side exhaust but can be schnerle ported, no tuned pipes but I would think a tuned one piece muffler like a Dynathrust is legal.
No retract gear....if equipped must be flown with them down. You can change tri gear to a tail dragger set up.
Can use a pump on engine but no oversize carb.
4 strokes up to .91 but cannot be supercharged.
You are right about the 4 strokes starting to dominate....a pumped .91 OS in a model like a Daddy Rabbit is a heck of a competitive plane....and the Daddy Rabbit is around 30 years old......bet it would be better in something like the Curare.....hmmmm....but then I don't like 4 strokes.
PilotFrog