I did a diff between the configuration_Adv.h files of the Rambo firmware vs. the Ramps firmware. These images are the laser related result (Rambo on the left with laser enabled and Ramps on the right):
In addition, you will need put these two defines somewhere:
#define SPINDLE_LASER_PWM_PIN 45 // Hardware PWM #define SPINDLE_LASER_ENA_PIN 31 // Pullup!
You can put them at the top of configuration.h or in the pins_RAMPS.h file. These pins are on the AUX-4 header. Not sure if these are all the changes or not, but it is the obvious ones.