> Does anyone know here I can get flags and a tonnaeu cover?
You'll probably have to make those. You might borrow them off a track or truck but they'd still need to be modified to fit what you're doing.
> How do I convert a track model into a playable truck?
Give the track model a unique name. Then, in the *.TRK file under truckModelBaseName put the name you give it. Usually you have to raise the model a couple feet; how far is usually just a matter of trial and error until you like the height. And of course, fine tuning can also be done in the trk file.
> how to change the wheels on a truck?
In the *.TRK file under tireModelBaseName put the base name for the tires you want.