The SPA is what it is. They've been around almost 20 years - plenty of 'stuff' has been written about their rules. The BPA was founded to provide an alternative to the SPA while allowing more modern designs, retracts, pipes, etc. You can run a 2-stroke 60 with any of the performance enhancing mufflers (like the one Performance Specialties sells)in the SPA. Technically. tuned pipes didn't really get popular in pattern until about 1977. Retracts were around well before 1975. The SPA was founded by a small number of pilots looking to simplify pattern competition. They achieved their goal successfully and the interest level continues to grow. The BPA will grow as well.
-Will