I believe I read somewhere the SK engine is not a high rev engine, its power band is lower, like around 9,000RPM. So a Pitts muffler and pump are not really necessary or desireable. The engine will do fine under less improvements. Realizing the Pitts is there to stay, yes block up one port to improve the exhaust back pressure. Cap off the nipple on the engine plate to ensure no air leaks into the crankcase. The engine should run better but never expect it to be a screamer, it was not so designed. It will probably pull a big prop at lowe RPM with little or no objections on its part -- that's where its torque is maxed out.

See Ed Moorman's thread on the SK 90.

Good luck.

Chip