Get a set of 20 metric drills from .3mm to 1.5mm at a hobby shop
Then get a vernier guage. I got a digital one for $20
Measure the drill that fits the stock jet nicely. Thats the baseline.
It should be around 0.7mm assuming a HT.
With some solder and the bits you are ready for jet resizing.
Tuning however, is a whole other story...