get a standard set of pinions and drill them out with a 0.157/4mm drill
I just did this on mine and it worked great The metal is soft and easy to drill. I did it on a lathe, but a drill press will work too.
By the way, I was recommended to use the 19 tooth pinion which overloaded the battery (but not the motor). I used special brass sleeving to bring that pinion from 5mm to 4mm and it worked well. If you like I will be more than happy to sell you the 5mm pinion set: 19, 20, 21, 22 teeth with sleeving to convert the ones I have not yet converted.
But I recommend drilling the standard set. Right now I am testing the 17tooth.
keepin' it real