You can find an example of the GBA SP being overclocked at the Japanese site below.http://nds.jpn.org/jittsaini.htm
Basically, it works the same exact way as my instructions for the original GBA, but you just have less room for components.
You could make an external mod pretty easily by just soldering a wire to each contact of the Oscillator Crystal on the SP's board, and then running the wires out a hole (or two) that you drill in the side. Then you could attach a crystal to those wires to overclock your SP. If you want to make it pretty contact, just find a connector that has two contacts spaced about the same distance apart as the crystal's leads, and wire the connector so it just barely sticks out a hole in the SP's case. you could plug the crystal into the connector, or if you don't want to lose it as easily, you could make something larger that contains the crystal wired to the mating version of the connector you wire into your SP.
Placing two crystals in parallel is how I and many other people performed our overclocking mods. I don't know the exact circuit calculations of why it works that way, but usually the Game Boy runs at the speed of larger of the two crystals connected to the same contacts.