I'm actually in the same situation with my RC51, but instead of SP1, I bought the SP2 radiator for my SP1. It sure doesn't fit as nice as I would like, so I'm looking for a SP1 radiator. Are you interested in swapping? The left SP2 radiator that I have isn't brand new, but looks very nice and came off a 2004 I think.
Let me know and I can send you pics if you're interested in a swap.