THis is so easy you're gonna say "why didn't I think of that?"
Empty the main case and if it has the same size bearing hole in the front as you have on the mainshaft, then just thread the the assy into there from the inside, and then put the big outer snap-ring on from the input. Then flip the whole thing up so the tail is up, and you are all set-up to push or beat it out.
But if the front is too small then use the big hole, except you will have to fabricate a bearing support big enough to not fall into the hole. I torched out a little circle about 6 or 7 inches in diameter, from about 3/16 steel plate (whatever fit inbetween there). Then torched out a center hole. Then torched out a slot to make a sort of horseshoe deal. I just slip this horseshoe between the bearing and first gear, then set the assy onto/into the back hole, and she's all set up again.
Thick flat bars work too, but under first gear, taking them both off at the same time.