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The starter motor is on the lower right front of the engine. It is held on by 2 8mm bolts (12 mm hex) I would suggest checking a couple of things first however. First, I would check to see if you are getting voltage through the starter relay when you turn the key. You want to see the cable going to the starter motor get battery voltage when the key is turned. If you are getting voltage on the output side, the next thing I would do is pull the spark plugs out and try to turn it over with the spark plugs removed. If it still doesn't turn over, I suggest using a mirror to see the starter motor. Check to make sure the cable is tight. If everything seems fine, unbolt it and slide it out towards the stern of the boat. If the boat has spent time in salt water, there is a good chance the nose of the starter motor will be corroded and stuck in the engine case. If that is the case, you are going to need to spray it with a penetrating oil such as PB Blaster. It may take awhile for the penetrant to work, but it will. The only other option is to remove the stator cover and stator and tap the starter motor out from the case. What we are guarding against with all this caution is if you try to pry the starter motor out of the cases, it will break it off in the cases. Then you are going to have to do a lot more work to extract it.
IF you have any more questions, feel free to ask.