Get user velocity, multiply it and then set it back, like this:
PHP Code:
new Float:origin[3];
pev(iUser, pev_velocity, origin)
origin[0] *= 2 // it's x
origin[1] *= 2 //it's y
origin[2] *= 2 //it's z
set_pev(iUser,pev_velocity,origin)
But i don't understand exactly how it works. I made once like that and it pushed me down on surf, so in this case you shall multiply "x" by 4 ,and z with y by 2.
Just play with z,x,y and try to modify values until you get perfect surfing boost.