what is usually the combo for it i have an 18t and idk how many teeth does the slipper have and the take off is so slow...can i use a 15t cb from my savage?thanks

Stock spur is 62 teeth
stock bell is 12 teeth
18 is out of the park geared high, unless you have some sort of super torque motor, I'd stay in the 13-14 tooth range max.

man i dont have those in my tool box ..u think 15t is ok ?since i just got this 2nd hand idk if the stock spur is metal ?

Yes the stock single speed spur is metal
15tooth bell still sounds high to me, but it will work way better than that 18t
It will work, just you'll still loose some bottom end performance and burn clutch shoes faster thna the stock set up.