Code:
force.x = 7*(key_force.y + key_w - key_s + joy_force.y);
        force.y = 3*(key_comma - key_period + key_a - key_d);
        force.z = 3*(key_home - key_end);
        vec_accelerate(dist,speed,force,0.5);
is the part you'll need to change
force.x = 14* will obviously be twice as fast
or change it to a variable and have the user define it

Code:
var cam_speed
...
force.x = cam_speed*(key_force.y ...
...

void cam_speed_up(){
   cam_speed++;
}
void cam_speed_down(){
   cam_speed--;
}

on_x = cam_speed_up;
on_z = cam_speed_down;


hope this helps