If I had to make that decision on my own I'd mount the sensor further downstream in a straight section of header just upstream about 5-6 inches from where that header dumps into the muffler. Your pipe shape and available space around the header might not allow an optimum sensor location though. As a general guide on a single cylinder motor with a single pipe exhaust you'll want to insert an EGT or O2 sensor nearest to the end of the primary pipe.
Never insert a sensor into a bend if you can help it because the sensor will not be exposed to a uniform flow column. The exhaust gases attain their highest temperature and expansion rate, and maximum stabilization of gases right at the primary pipe exit. This assumes that the primary pipe length and diameter are optimum matches for the motor.
So, for example if you insert a sensor upstream midway in the pipe you'll get inaccurate, inconsistent and dirty readings in that underdeveloped column of gases. Its real dirty up there in most throttle positions/torque loads... its still consuming oxygen, still continuing combustion and producing heat and still expanding, so column density/distribution of gas' molecules is decreasing rapidly as the gas' velocity increases... thats a very unstable environment for data gathering.
Grab your data where its most stabilized at its maximum temp if ya can. You can find that sweet spot for data gathering in the primary pipe (single cylinder only, multiple primaries data gathering location is inside the collector) with an EGT sensor.