There are two sizes and the smaller ones might work OK for N scale. they come in two halves so you could glue them together in a channel shape rather than as a box girder. Half are solid and half are solid steel plate which would work for a newer (non-lattice) conveyor.
Perhaps the uprights can be made from the larger sections? For a more modern looking conveyor perhaps the Gusseted girders would be a better choice?
Another possibility is to build some pieces using some Plastruct girders. They come in a lot of sizes that would fit an N structure. There are quite a few different designs. This is a sample of some:
Conveyors are constructed a lot different today than decades ago. All our modern conveyors are simple channel iron with cross members and framed rollers mounted on top and return roller under. The old style truss type conveyors were a lot more complicated and maintenance headache.
Google Metso equipment or Lippmann for some pictures.