Pokey in pieces; broken, purple pinion gear
Pinion gear crack is on left, runs parallel to shaft
Fortunately, Tamiya has an inexpensive replacement set of plastic and metal pinion gears, TAM15289, Mini 4WD 8T Metal and Plastic Pinion Gear Set. Cool. The set I ordered arrived a few days ago and after installation, Pokey's geartrain is restored to full, smooth, silent health. Next up, equipping the plucky little robot with bluetooth remote control and telemetry capability. Articles to follow.
Incidentally, there are other pinion gear alternatives. You'd need a 48 pitch, 8T pinion gear for 2mm shaft. I don't know how to measure pressure angle or pitch diameter, so I guess you'd have to experiment such as with gears from this web store or maybe this one.