Hi Jonas , to remove the starter motor you will need to drain oil OR figure some way to lean your bike over to the right as you will need to remove the left engine cover to access the starter mounting bolts ( one of which is got at via a hole in the starter idler gear) . It is possible to remove the end cap off the starter without removing starter from engine. Before you do any of that , try putting bike in top gear and roll it backwards , the idea being to rotate the starter motor as they may be a bit of crud on the comutator ( the bit with all the copper segments that the brushes run on )...........might work
.Depends on how keen you are , I would remove the starter and strip it down ,( paying particular attention as to how it goes together as its possible to get it wrong and the starter will run backwards ) and give it a good clean out and inspection