As forum member Knotty Rope has told you The engine has to come out to replace or service the starter. The bolt's that hold it in Place got up through the Oil Pan, so there really is not a way to get it off the engine without Removal of it. I don't know why John Deere did not Put Two hole's
in the engine cradle Plate that is welding in Place between the frame rails of the 316's & 318's so that one could remove the starter attachment bolt's on an Onan engine ? It sure would have ben a good idea and would save a lot of time and labor to do so.
If you need a replacement starter I would suggest that you get a hold of forum Onan Expert Boomer.
They do sell a Re-Pop China made starter for the Onan's But, from what I have heard they are Short Lived and not dependable. Here is his contact information, He should have a Factory OEM Onan starter for your engine that will last much Longer than the Cheaply Made China Knock off one. When replacing the starter on an Onan engine be sure to check the starter gear to ring gear lash as an Incorrect lash setting in that area will cause Pre-Mature starter failure. I hope that it help's, Kurt