GBA save wipe?
 

I have just picked a dirt cheap gameboy advance sp with mario kart, but the cartridge already has everything unlocked and beaten, is there any way i can get rid of the saves and start from scratch?

Yeah, it's the top L button, right on the D-pad, and Select. Hold those down when restarting and it'll prompt you to delete saved data.

I had to do the same thing when i bought Advance Wars used.

Cheers dude!!