That URL indicates most of the bearings are shielded. i don’t see any point if the box is oil lubed.

What is the casing material? I might suggest heat and cold be applied as a means of fitting the pocketed bearings, if appropriate.

If the bearings are a loose fit in the casing, clearly one would fit to shaft first. Bearing-fit loctite, or similar, would be my choice if the housings are that sloppy.

In that case, the easiest way to reassemble might be to put the shafts and bearings into the blind bottom holes first. Then heat the top with the open holes and place it down over the bearings already installed on the shafts. If the holes are open you should be able to fiddle the bearings into line with each hole.

Freezing bearings barely reduces their diameter. The temperature drop is only 20-30C. Heating the cast iron casing to 100 to 200 C gives much more expansion and is the more common way of assembling gear boxes. Try first at 100C. (Heat with propane torch until you can spit on it and the spit really sizzles as soon as it hits. Or use the oven while the domestic authorities are out shopping. )

Shielded bearings are sometimes used in transmissions (including Harley Fergusson motor bikes) and should work just fine. The shields have a small gap at the inner race so oil can get in there but the shields tend to hold a bit of oil in the bearing, and maybe keep a bit of grit out as you say.

Simply put,

When installing a bearing into a housing, pressure needs applying to the outer race….never apply to the inner race.

When installing onto a shaft, the force needs to be applied to the inner race, not the outer race.

I recently stripped and rebuilt the gearbox on my Warco Super Major version of the RF45 primarily because of leaking oil seals.

The Graetech article was invaluable

Having thoroughly cleaned out alI the gunge I replaced all the bearings and oil seals with good quality branded ones

I found that all the bearings could be removed without the need for heat or excessive force – maybe I was lucky

Unsurprisingly a couple of the bottom bearings were shot due to grit

When re-building I found it quite straightforward to do so from the bottom up, finally dropping on the top cover with gentle tapping

The result is a transformed machine – much quieter and smoother and the sight glass shows the oil to be remaining beautifully clean

What a pity the manufacturer would not spend a few minutes cleaning out the castings in the first place

PS I took the gearbox off for the rebuild. If I have to do it again I don't see any reason not to do the work in situ

Just with a paintbrush and some dirty diesel I had

I took lots of photographs with the intention of doing a write up, but the road to hell is paved with good intentions and I am well along it

The section I really should put up is the power downfeed which Graetech's version didn't have