Hook up a timing light one cylinder at a time.----Might help pinpoint which cylinder is the issue.-----Might be as simple as a bad sparkplug.----Or bad sparkplug wire.---Or loose connection / broken wire.
What is the total; $$$ amount spent so far with the parts cannon?----What actual trouble shooting has been done so far ?-----Revving on a hose in gear is still neutral and can be misleading you.
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.