OK, in the Honda shop manual you definitely have to pull the primary shaft before splitting the cases, and reinstall it after reassembling the cases.
This doesn't mean you can get the starter gear and clutch outer OUT without splitting the cases, but it looks like you can. There's a 25mm spacer that will allow the primary chain sprocket to move over enough, assuming you have room to get the chain off the sprocket.
It's pretty hard (umm, impossible?)to do with from below with the engine in the frame, you could remove and invert the engine or just flip the whole bike upside down (drain oil and remove the battery and fuel tank and figure out some way to deal with the front brake reservoir first).
Remove the oil pan and oil pickup.
Remove the oil pump and the clutch cover and clutch pack. Remove the 12mm bolt holding the primary drive gear and its washer and the drive gear. Remove the circlip, and the ball bearing and 25mm spacer should come off the end of the primary gear.
Pull the primary shaft out the oil pump side.
Work the chain off the sprocket and shift the sprocket as far as it will go towards the clutch side.
Now you may be able to slide the starter gear out of the starter clutch outer ring and remove it. There's a roller bearing assembly, hopefully the rollers are captive but they may come loose if the cage slides out of the gear centre hole.
Once that's out you may be able to work the whole sprocket/damper/clutch outer unit out of the case, otherwise it's going to be a bugger. You will need to inspect the clutch outer I think (if it's there!) and it will come out of the sprocket along with part of the damper but the 8 rubber damper blocks are going to come loose as well. Much better if you can get the sprocket/damper/clutch outer all out as a unit. Looking at the photos, I don't think it will come out though. I have a loose engine (400) here but I don't have the time or energy to try it.
The only major part that could be left out and have the whole primary shaft workable is the clutch outer ring, hopefully you are just missing the rollers, springs, and spring caps.