you can always try searching for a smaller front sprocket. simply put (no pun intended), 1 tooth on the front sprocket is roughly equal to 2 teeth on the rear sprocket. if you are unable to find a larger rear sprocket, you can look for smaller front sprocket instead.
i am sorry bro, Suzuki GS-150's front sprocket bay cannot accommodate any sprocket larger than 14 teeth. I could be wrong but I remember stock sprockets as F14/R43. like I said, I could be wrong.