Front one i bet... it gets the load from the belt.
Careful getting the front nut off. Sometimes they seem to flog the threads, maybe to lock the nut on.
you may need to grind the shaft end to fix it.
Pull the rear cover and put armature in vice to undo the nut. When rear cover is off, you can see brushes. There is a service length listed for them, but they prolly ok, and easy enough to change later.
the brushes are carbon. So easy to file them to size if new ones are a little big.