You can true-up the plastic gear, by applying light-pressure to the high-spots with a piece of hardwood. I bought 3 main gears, 2 from one supplier, one from another, all of them were junk right out of the package! Not a one of them spun true.
I used 2 (1.000" gauge blocks), on a flat surface (kitchen table), I draped a piece of white paper behind the gear (makes it easier to see). Using a white grease pencil I marked the gear (on the spokes) to indicate the "high-spots" as I rotated the gear on the blocks, after I found the 2 high-spots, I started applying light pressure with a block of wood, at those marks, rotate the gear 360 degrees still you don't see anymore wobble.
You could also spin the gear while it was pressed into the shaft, if you rested the shaft in a V-block, use a framing square as a true 90' sight-line, to sight in the gear.
Above and beyond would be using a .0001" dial indicator (with at least .025" of travel), while the gear sits mounted to the shaft, and the shaft spins in a "St. Mary's Gauge," with shaft runout less than .0001" at 1" away from steady wheels. Wishful thinking with plastic parts[].

Shadow