[gregp]

Most converted dirt bikes that I have registered and inspected (and I have done many) have been checked for a mirror, a horn, a high/low beam DOT headllight, *both* front and rear brake lights, working directionals, DOT tires, a plate light, and a keyed ignition. Some of the things I have been rejected, or been given a hard time about are: lack of a chain guard, *buddy pegs!*, a speedometer, and having the ball broken off of the levers (unless you have bark busters).
I have never been hasseled about a plastic tank. Besides, KTM's, some Ducati's, and Buell's have plastic tanks.
The most important thing, of course, is that your VIN must match your registration.
The level of scrutiny varies from place to place and also often varies greatly amongst the employees doing the check.
I always assume the worst, prepare well, then remove half of the stuff when I get back home, box it up and save it until next year.
I have had a fully inspected CR500, CR250, KDX 200, several KTM's, and a couple of Husky's, and may now work on getting my Beta REV 3 street legal. I got lazy eventually and just went out and bought a DRZ400S.
You probably will not have too much trouble with an XR unless the exhaust is obscenely loud.