All bearings and drive gear are standard proprietary items that can be obtained from either Magnapower or most bearing suppliers.I hope the problem with this sentence is obvious to everyone, but if not I'll give you a clue.
Something that is standard is usually well understood and can be made by different people. For example, screws are standard. They come in set sizes and lots of people make them. Screws from one manufacturer are very much like those from another.
Something that is proprietary is usually shrouded in mystery or secrecy in some way. For example, drugs are often proprietary. They are developed (at huge cost) by a company who then patents the drug allowing only them to manufacture it for a number of years. The patent will be written to make it as difficult as possible for a competitor to copy even if the law allowed them to.
So how can a replacement part be standard proprietary? It is either standard, at which point you could buy a replacement from a number of manufacturers, or it is proprietary and you will have to buy it from the sole company who owns the rights to the design. I can't really see a middle ground.
I'm guessing that what they mean is that the parts are proprietary but that they are so common on their machines that most companies who sell bearings and drive belts will stock them.