Upgrade Sprinter 906 starter

I changed the battery with my Sprinter first because it partly bad sprang up and it also sounded so slow starting, as if it weakens. With bridge no problem. 2 weeks after that also the Lima failed, not always, but again and again. Lima lamp in the cockpit and no charging voltage. Also this was changed and since then is quiet. But, the problem with the weakening starter persists. If you start e.g. several times in a row then the Anl If you take a vehicle to bridge then you can’t take only one battery to bridge then. After 1-2 hours it then starts again without help. I checked all the cables as far as possible, but can actually also exclude because battery is connected almost directly to the engine. I would now like to expand the starter to check the contacts and/or to replace the starter. Can someone tell me how I rec Is it easy to get the starter? It is the normal 4 cylinder 2.2 CDI