I don't know where you can get Yanmar service manual, hopefully someone will chime in and let you know, I wouldn't think it's to hard to take starter off, just be sure and disconect positive side of battery before you start, when you have it off make sure you can move pinion gear up, it will twist to move it up, put a little electrical grease on pinion shaft.
Look at pinion gear and flywheel gear where starter goes into engine for any damage to either one.
After you lube pinion shaft and if no damage found put it back together and see if that helped.
