Hi
The SPI functionality is baked in to the ATmega328 design. It sits on pin 5 of port B, and PB5 is addressed in Arduino-speak as Digital Pin 13. It will be far easier I expect to move whatever alternative functionality you have in mind for D13. What is it that's causing you this issue ?
If you're up for more reading, check out the ATmega328P datasheet, and these functions are summarised in 13.3.1 Alternate Functions of Port B, table 13-3 on page 82. For a handy reference to these, I keep this reference page from adkafugu within arms reach.
Cheers!
Geoff