Starter Motor Zane Twin

Regarding brushes generally, these can be shaped to suit with sandpaper etc. Introducing a curve at the end is nice but will happen anyway with wear. Contact is only through one pair of opposite segments at a time. When installing new brushes they can take a little time to bed in regardless of a pre-made curve, professionals will usually apply a little abrasive “comm chalk” to help make the best possible contact.
More important is to ensure that the brushes fit nicely in the holders so they slide easily without being loose, and the springs are able to hold the brushes straight and firmly onto the commutator. Brush holders should be clean and dry and fixed firmly.

If the starter works sometimes but not always (perhaps you need to rock the bike in gear) and lacks power then I would check the commutator carefully for any signs of a dead pole, is any segment showing more wear, arcing, or colour change. If you find one then you may also notice similar signs on the opposite segment. Also the old brushes may show arcing and chipping on the trailing edges.
If it looks ok then clean the commutator with sandpaper before reassembling and running it on the bench. It should sound fairly quiet and run fast and smooth. If it sounds like a small arc welder then it’s probably time for specialist help and maybe a new motor.
There are several other tests that can help but also require specialist knowledge and tools.

Please note I’m not a zane expert, I’ve just fixed a few electric motors 😁
Thanks for that.
A test I saw the very experienced Auto electrician do to my 3c stater motor after he rebuilt it was to hold its drive gear against a soft pine heavy piece of timber and energise it causing it to rub and cut into that timber like a router. That proves it produces strong power and does stall under load. Looks dangerous as hell and surprised me at the time.
Yeah. It's an old school test. Sounds crude, but if a starter motor can't do that, it ain't never gonna turn an engine over.