These threads drive me nuts.
Imagine you had an 82/83 motor in bits on the bench, and an 86+ motor in bits on the bench, and all the parts were jumbled up.
1. Any part that is identical or cannot be reasonably identified as 86+ is legal for pre-85.
2. Any part that cannot be seen from the outside is effectively legal, even if its different* (subject to the usual restrictions to the number of forward gears, and the bike being entered in the correct capacity class).
If, for example, the 86 has a higher compression piston or bigger cam, then that's 100% legal as the internal parts are essentially free.
If, for example, the pre-85 motors were only 5-speed (they weren't), then the later model 6-speed would not be legal.
If, for example, the 86+ motor has a different head casting with more fins or something, then that's not legal (but highly likely to be ignored).
If, for example, the 86+ motor has a different head with a centre port rather than a side port, then that would not be legal.
If, for example, the 86+ motor had a longer conrod and a taller barrel, then the longer rod would be legal, but the taller barrel would not.
*This is not 100% technically correct, but it's how it works.
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Ignitions are specifically allowed to be swapped from points to CDI, with the only real condition being that the motor must appear era-correct from the outside.
