A good weight for you is 136, because you're thinking about this backwards.
Your BMI is in the normal range. You don't give your age, but you sound like a 15-year old male. If you reduce your intake of what your body needs as a teenage athlete - calories and nutrients - your times will get slower and you will get injured.
Turn the question around. Instead of "How fast can I run if I lose weight?", the question is: How will your weight change if you run faster times in XC?
If you raise your mileage this summer and do all the other things that smart XC runners do, maybe your weight will drop, or maybe it will rise (because you're a growing teenage athlete), or maybe it will stay the same. And that will be the ideal racing weight.
Save this dieting crap for when you're middle aged and slow. Throw away your scale. Just get out there and run, man.