Is modification to a COTS part allowed?

[GeeTwo]

If your 3-d printed hubs do not require you to modify the COTS wheels, they're still COTS. As they are not actuators or control systems, you are pretty much at liberty to modify them as needed. However, if you drill or cut on them, that makes them fabricated items rather than COTS. The only difference between COTS and FABRICATED items is that fabricated parts must be made after the start of build season (a few exceptions, bumpers, and attachments), and are not exempt from bagging/withholding rules (an even smaller set of exceptions).

[GreyingJay]

Tires = COTS

3D printed hubs = fabricated item

Put together, you have a fabricated item (assembly).

[rich2202]

Tires = COTS
Gear box = COTS
Motors = COTS
Control System = COTS
Chassis = COTS

Put them all together, and you have a fabricated item.