That bolt threads into the exhaust housing to hold gearcase at the back.-----Obviously that will need a heli-coil.-----There MUST be a local shop that can sort this motor out.
There are 4 bolts ( item #66 ) he has 3 of them out with 1 broken off.------Now the one that is broken off could be jammed with aluminum oxide.----Heat can be applied there to possibly break up the corrosion products.
You have to heat the aluminum that the bolt threads into !-----Aluminum expands ( 0.000011" / per degree F / inch ) at twice the rate of steel.----Therefor the hole and threads get bigger and let go of the bolt.