You can often find sprocket covers on eBay, if not, I may have a spare you can have, but it lacks the clutch actuating mechanism.
Chemical metal may be enough to fix the broken parts of engine case or you might manage it with Lumiweld, but it's hard to judge without a photo.
I can’t see the point of fitting bigger pistons and cylinders, the clutch isn’t capable of taking much more torque without going for uncomfortably heavy springs and it’s not really what the Fazer is for. If you want a faster bike, buy an R6.