Maybe you have that diagram. But I’d think that main pin should be able to be ‘punched’ out. If it’s rusted in, try all your favorites, WD40, even Coke a Cola, and then perhaps drill it out….sorry can’t be of more help..
I don’t remember a pin in the gear box when I overhauled mine last year. There is a key at the bottom of the engine crankshaft, so take the bit off, remove any burr around the hole that hold the bit to the shaft and the shaft and gear should push up out of the bottom white housing.