On the subject of your motor Croftie, if you isolated the burning smell to the brushroll, the carbon you see in the post motor area isn't that much. Carbons slowly wear over time and a little dust is to be expected.
Stick some new filters in (or wash the old ones) and wait till the motor conks out of its own accord.
PS - Nice rivet job somebody did there. Looks like it had a motor some time in the past anyway.