So, I have a '03 and when I bought it the left radiator was damaged, so I purchased a new one and mistakenly bought one for an SP1, is the swap possible? And if so, what mods do I need to do?

Pretty sure the radiator itself is the same, but I think the fan, mounting points and electrical connector for the fan are different...

Can I make an SP1 radiator fit an SP2, though?

Yes you can it will mount on the frame just as the SP2. You will either need to purchase a SP1 fan or have someone weld tabs on for the SP2 fan. You will also need to block the temp senor port as the SP2 runs it off the thermostate housing. I will say if you run the SP1 fan it is larger and will cool better.

Made the attempt, due to the sp1 fan being larger, it would not fit to the frame, as the housing is also larger, I tried mounting my fan to the SP1 radiator, but it was still a no go, because the fan housing for the SP1 placed the fan farther back on the radiator. And, the SP2 fan housing wouldn't fit on the SP1 radiator's fan housing mounting tabs.