If anyone has an unserviceable Bosch starter motor or parts please see the Wanted section of the forum as I need a replacement brush holder plate.
Did you try this?
If mine was playing up, I would give one of these a try…
https://www.ebay.co.uk/itm/Starter-Motor-New-Moto-Guzzi-T5-850-V-1000-G5-V11-V35-V50-V65-V75-D6RA21-Top/183512322574?hash=item2aba2fd20e:g:AcYAAOSwcwdck2nm
Brand new, £58 delivered to your door.
Edit, if you need any hints on stripping and rebuilding, there is a great explanation on This Old Tractor …
https://www.google.com/search?client=firefox-b-d&q=this+old+tractor+guzzi+rebuild+bosch+starter
I see the motor on eBay is now listing at £73.50 but since Brexit it is also likely that HMRC will bang on an import tax.
My local auto electrical supplier is ordering me an upgraded replacement for the Bosch at £72.