There are two things to keep people from breaking the 1986 rule. Their conscience, and the S/N on the motor.
I would hope that the former would be sufficient, but the latter would be easily verifiable.
As for the point of classes that you can run in. You would be able to run in Twin, Pushrod, or Classic if it wre pre-'86.
That is the "let those who ride decide" paradox.