Whirlpool produces a kit of parts that they call a maintenance kit,
which is for their basic dryer design. It includes a belt, idler
pulley and--iirc--two rollers. My question for you is, why do you feel
that you need to do this? The belt doesn't need to be replaced unless
it is extremely cracked, and even then you can wait until it breaks.
The idler almost never needs to be replaced unless it makes noise. And
same for the rollers. Why not let yourself off the hook and just
forget the whole thing?