The rule that covers it all is the seldom enforced 14.2.6.1-A Engines must remain externally unchanged. In reality it's a difficult to enforce rule when you consider that fitting an aftermarket head or a reed kit actually changes the external appearance of an engine so therefore it's illegal. That's silly of course and considering that folks have been chopping fins for half a century, I wouldn't disallow Wheelsmith style finning if I was eligibility scrutineer. My squarie 250 barrel shown above is an example of what folks have been doing for years.
Mark In the 2013 MOMS Chapter 14 is one page and it is the year in pictures. Chapter 18 now covers Classic MX & Post Classic MX & DT. For this weekend the rule you quoted is cover by 18.5.1.5, 18.5.2.4, 18.5.3.3, 18.5.4.3 & 18.5.5.3. Yes there are other rules that need to be addressed also.
Unfortunately some officials like to cause trouble. As an example there was an official in NSW who decided that because the use of starting blocks was only in the GCRs as a junior rule they weren't permitted for senior MX. As this would be discrimination against those who are vertically challenged and the fairer sex, there is now a rule in the MX section 17.8.1.8.
But as someone said to me KISS (kept it simple stupid). Rick Doughty said to me by email "Keeping things simple is also important because the more rules you have the more rule breakers/cheaters you will have."