Depending on what is actually included in the "toner cart" buying a new one may not fix the problem. Shaking can help distribute the remaining powder in a nearly empty cart, but it should not be a problem unless you house sits on the side of a hill!
However, the results you are reporting don't seem to indicate a lack of powder. That would be seen by
unprinted streaks on the output. What you are seeing is more an indication of that area of the drum being heated too much or the transfer drum not being cleaned properly. Most laser printers have separate drum carts and powder/toner carts. The toner carts are relatively cheap but the transfer drum carts are much more expensive since they have a highly polished and symmetrical drum(s) that transfer the image via static charge and allow the fuser to heat fasten it to the paper under pressure. However, on this rather ancient model, both those operations might be built into one expensive part. Of course, one must expect to pay more for certified antiques!
OTOH, it may be as simple as a worn/broken 'scraper' that is supposed to clean any unused toner off those drums. A new putty knife might be all you need!
And you might be able to clean the tiny heater wire, especially on that side of the drum. Might be best to turn the printer OFF before touching that wire, assuming you can even find it.
