starter motor refurbishing

So I have decided to have a go at refurbishing my starter motor.
I have bought a used one to put on the car so the one I have taken off I have started to strip it down and see what's what.
I'm by no means technically minded but I have a lot of common sense haha
So here we go. Hopefully someone can guide me along the way.
The solenoid was working okay. The starter motor just wouldn't or couldn't turn the engine over.
Some times it was slow to turn over and other times it was fine. Then it just couldn't turn it over.
I have removed the starter motor.

I removed the cap where the brushes are and found they had worn down and 2 were worn pretty bad.

The magnets inside the casing were covered in carbon mixed with grease so I have removed them and cleaned them and refit them.
I didnt take pictures of this tho because my hands were caked in the stuff.
I have just ordered new brushes

Everything else inside looked okay.
So im hoping it was just the shit that had built up inside and the old brushes worn down that was causing the motor to stuggle to turn.

Any input is greatly recieved!
Is there anything else I should swap while its all apart?

checking condition of commutator the brushes run on would be good & the condition of bearings, failed windings can cause slow or partial spinning symptoms, check solenoid switching for high resistance issue .
If lucky it brushes & dirty commutator or solenoid high resistance .
Always worth a cheap overhaul attempt as many times it all that needed. Good luck & have fun :-D

Thanks MrB
The thing the brushes run on looks in good condition.
I will remove the solenoid and give that a check over too.
Hopefully the small overall will work and give it life again.
If not I havent lost anything.

Is the brushes normally the thing that commonly goes on them?

Couple of useful tips other than what's been said
1 use sand paper to clean components like comutator as emery paper and the like will cause shorts as contains metal particles
2 check action of throw out dog gear and diff gears, a clean and regrease should be fine
3 bench tested starter unit prior to re- fit