I have rebuilt one and it gave me another 6 months life.
Normally its where the wire goes into the metal body and the rubber gromet becomes brittle.
Make sure you recharge the ATF before you start stripping it down as sometimes that can be the fault. If its not then strip down, clean up the electrics and tidy up the brushes.
You can just about do it on the transom but you'll need to slack the engine mounting bolts to give you some play. (Watch you head and language whilst working under it!)
If that works enjoy but be prepared to buy a new one fairly soon afterwards.
When you get that far contact Arco Starting and Charging
in the US as they make a better one without the dodgy rubber gromet seal and its cheaper than the Merc one which is about £600. They sold me one for about £250 if I remember rightly but don't forget the import tax on top when it lands in blightly.
I've got the Optimax service manuals which you can download for free from our website: Northern Exposure Rescue
If you need any more info give me a shout and I'll see what I can do.