If the gear is smaller in diameter but teeth are same exact size and depth and shape, you'd have to remake the clutch so the centers of the gears are moved outward far enough for the gears to mesh with the ring gear. Then you'd have to remake the axle so the sun gear is now large enough diameter to reach the planets.
If the gear is the same diameter the teeth will be different sizes, depths, and shapes, so you'd have to remake the axle and the ring gear to match those.
I don't know what the new gear ratio would be, but could be significantly different, causing the wheel to run at a completely different speed than it used to, and load the motor completely differently.
It's very likely that the amount of work and cost of all the parts will be far more than a whole new motor.
FWIW, even if you find gears with the same number of teeth, they need to be the same shape and depth as the originals or they won't mesh correctly with the sun and ring. If they have bearings built in they'll need to have the same ID as yours, or you'll need to alter the clutch mounting points for them to fit. If they don't have bearings built in, you'll need to order bearings with ID to fit the clutch mounting points and OD to match the gears' bearing holes. .
Can you post some pics of the wear on the gears? It's likely that only the (presumably) plastic planetary gears would wear, and not the (usually) metal sun and ring gears, but it's a good idea to verify that you don't need to replace them all before getting new planets.
Is there wear on the clutch, too? (meaning, does it have trouble engaging under higher loads, or does it slip?) If there is, you might need to replace it along with the planets, though there might be things to be done to extend it's life depending on it's design.
