I've just replaced the steering head bearings on my newly bought Commando and the spacer tube was loose when I took the old bearings out. This explained why the races had become so heavily indented, that the steering self centred in the indents and was ready to tank slap at the slightest provocation. Although this made for a very engaging ride I thought this had to be sorted out before it sorted me out.
When I tapped in the new bottom bearing in to what I thought was 'home' the spacer tube didn't extend past the recess where the top bearing sits so I would end up in exactly the same situation as I was before. If I'd just clamped the whole assembly up with the top yoke there would have been undue pre-load applied to the bearings causing them to fail again very rapidly. Using a drift tube I'd made from the old bearing outer race I decided to give the new bottom bearing a good smack with a hammer to make doubly sure it was fully home. It took a bit of force but this time it was fully seated and now the spacer tube extended a good few thou past the top bearing recess. Installing the top bearing so that it just 'nipped' the spacer tube, the top yoke now clamped on and fully assembled the Commando now handles like a featherbed. If I was doing this job again, I would make doubly sure that the bottom recess was meticulously clean and free of old paint or powder coating before attempting to install the bottom bearing.
With regard to fitting taper roller bearings, I don't see how this could possibly work with the standard yokes, as the Commando yokes don't have any means of adjusting the bearing pre-load between the yokes.