Hi Muz,
Firstly welcome to the forum, great to have a Scott Bonnar Clone on here as it's a great comparison between the originator and the imitator.
This is a perfect example of Chinese steel quality compared to Australian made steel. Really this shaft should not fail at such a young age as it has. I guess this is why reel mower purchasers have hunted down genuine Scotty's opposed to buying a imported unit. But as I always say "it is what it is".
Now as far as the job at hand is concerned,
You are going to have to get the original bearing out of the housing as you are wanting to use the original housing again, so, I would remove the drive gear from the end of the shaft and then remove the two mounting bolts and remove the housing and shaft together as one assembly.
As your machine is rather newish the gear should slide off fairly easily. The only issue I can see is getting the nut undone as you've already removed the engine. So you will have to use a set of vice grips and lock them on to the shaft so it can't slip. As you are not going to reuse that shaft don't worry to much about putting marks into it with the grips.
From there you'll have to use a vice and open it precisely enough so as to rest the edge of the mounting surface of the housing over the top of the edge of the jaws of the open vice on the housings outer extremities, but just enough clearance so as not to restrict the bearings movement downwards as we want, thus the engine end of the shaft should be vertical now pointing towards the roof . As you are no longer going to be using that bearing and shaft again you can use a soft blow hammer and tap on the end of the shaft which should push the bearing out of it's housing downwards between the open jaws of the vice. As this machine is relatively new the bearing should come out rather easily. It's somewhat harder doing this if it's in an old Scotty that's been sitting out in the weather for many years and corrosion has taken a stranglehold, but that's not the case here.
Now as far as reassembly is concerned all you have to do is reassemble the new bearing into the housing remembering that you must not apply any pressure to the inner race alone, only the outer edge of the bearing. You can use a vice here as a press to push the new bearing into the housing but I would use two nice clean and perfectly flat pieces of MDF sheet timber to protect the surfaces of both the bearing and also the housing surfaces. **Please note, if it's the the same as a Scotty the bearing will be pushed in past the mounting surface of the bearing housing, thus you'll have to find a general half inch drive socket of the same size as the outer race of the bearing so as to push the bearing in fully sub-surface, remembering not to place any force on the inner race. Before you do all of this work you can put the new shaft into the freezer making sure it's in there for a good half hour prior to requiring it and when you're ready you should be able to slip the bearing straight over the end of the shaft by hand and allow the shaft to normalise. This should be able to be done without pushing the bearing backwards out of the housing. Maybe it's best that you take the housing with the bearing in it to the freezer and do this small part of the process right as you take the shaft out of the freezer. Once it has normalised then just install the full assembly back into the mower.
I'm only explaining the freezer process here as you probably have rather limited tooling. I personally would use a different method of installation but I have much more engineering tools at my disposal I guess.
Procedure is now just the reversal of the disassembly process.
Hope this helps.
Cheers,
BB.
