I have a camvac, had it about 10 years although it is just the single motor version.
It sounds like it is a supply issue rather than your motor. There could be several causes for the sparking and intermitent functioning of the motor however.
It could as you suspect be the brushes causing the problem, if you do need to change them be sure to change the brushes on the other motor at the same time. However, before you do that, consider continuity problems caused by poorly connected or frayed flex or wiring.
I am not an electrician but simple jobs are within my skill base. If it were me I would check the connections at the plug first, the constant "humming" of an electrical supply can and does vibrate the terminal screws loose, not to mention pulling the plug from the socket. Also check the terminals at the motor end, ensuring they are tight and there are no cracks or fraying to the cable sheathing. I had to make a repair to the rubber grommet where the flex entered the machine. The rubber split exposing the wiring beneath. Next, check for continuity of each wire in the flex from the plug to the motor, you will need a small test meter for this though (or a simple car bettery bulb with a couple of probes powered by a 9V battery), it is a very simple test, just set the meter to the lowest Ohms setting and connect a probe to each end of your wire within the flex (eg live wire at the plug and motor end) be sure to remove the ends of the wire from the plug and the motor terminal for an accurate reading. If you have a reading you have continuity, no reading means the wire is broken or frayed somewhere along its lenght, so just replace the flex then re-test for continuity, then test the machine. All these checks can be done by you and don't cost anything. Failing that, pull out the brushes. If they are worn it will be apparent. If they are worn, you should see the base of the stud showing through the base of the carbon on the brush, they will need to be replaced. Again a very simple job and not expensive.
If these checks are done and all found to be correct you may need to have it professionally checked out. I took a mitre saw (12" Bosch)in for repair recently. It was £22 to have the machine checked and the faults identified. Should you proceed with the work that initial fee of £22 is taken from the total repair billing. I would suspect the repair may still be worth the cost compared to the expense of a new camvac, which are of course expensive machines.
Remember however, if it is an electrical fault don't take chances, sparks and dust do not tend to have a happy ending.
With regards the brown paper bag. It will work without it, but it is there to protect the motor. Mine has the same bag as when I bought it, including the original rubber band which secures the bag to the cylindrical motor. Not very high tech, which means it can be easily replaced, a brown paper grocery bag will suffice.
Good luck with it.
This is when you discover you have counterfeit bags !! :wink:
