This sort of situation is really impossible to treat fairly under Portsmouth. You've changed the boat too much for any base class + modifications number to be realistic.

Based on what you said, I would start with the 18SX number (71.3) and take a JU (oversize jib = .995) and MN (non-class legal main = .995 ) for a D-PN of 70.6. I think that's a pretty generous number, considering that the stock TheMightyHobie18 has a D-PN of 71.4.

There's no modification factor for a undersize spinn, but if it's not at least as fast why would you use it instead of the class-legal one?

The scorer might also start with the Hobie 18 base number and hit you for a JU, MN and SP, for a D-PN of 67.9. I doubt the boat would sail to that number.