Update on using a motor with 1/8 shaft... pinion gear options...

LC Racing pinion is a direct fit just need a shorter set screw. comes with a 3mm, need 2.5mm
Hot racing pinions will work but the hole of motor mount and the hub of the pinion are like exactly the same size so they rub. but a quick little massage with the dremel to open up the motor mount hole and they work. also requires a shorter 2.5mm set screw.

LC Racing 16 tooth p/n: L6206
Hot Racing 18 tooth p/n: CSG18M05
Hot Racing 15 tooth p/n: CSG15M05

any m3 x 2.5mm set screw will work I bought some losi (yuck) p/n: TLR6288 because it was the first i found