I'd be hesitant to use a brush cutter attachment for a quad. I think sharp stubs of brush could likely too easily puncture tires. One would be ok for mowing jobs. I can't see it as a haying machine.
It looks like the Baja 90 ATV has been dicontinued. I just checked their website. Here is the owner's manual I just added to the library if anyone needs it.
As far as part go, I'm finding a lot on Amazon that fit a wide range of Chinese atvs. I searched Baja 90cc ATV Parts