Removing the wheel bearings is a bit of a faff – there is a spacer between the two bearings that will move from side to side slightly, if you poke a suitable metal punch through one side so it connects to the top edge of the spacer then, with some judicious tapping, you can gradually knock the bearing on the other side out.
It took me a while to figure this out and in the process I damaged the inside of the spacer on the rear wheel, so I replaced it just in case. The new spacer design has changed from the original – perhaps the new shape makes it easier to pivot the spacer when it is in place so you can more easily get your punch on to it?
It is a good idea to heat up the rims before attempting to remove the bearings as the are an interference fit (a low mess way to do this is to wrap the hub in a towel and then poor on boiling water). You will inevitably damage the bearings when you remove them so make sure you have replacements ready.
The 1978 z2 is fitted with four 6301 bearings: 12mm (inside) x 37mm (outside) x12mm (width). Note that the front wheel is a different size to the rear on later model bikes and they use different bearings.
The original bearings had rubber seals on one side only but I could only find bearings with seals on both sides (“2RS” bearings as they are known in the trade). Apparently a single rubber seal was always used on wheel bearings on older bikes and the hub the filled with grease that melted with the heat generated by the brakes so that the bearings were kept continually lubricated. You can prise of one of the covers if you want to keep it per Honda’s orginal specs but the lubricants in modern sealed bearings are expected to last the life of the bearing otherwise.
The 2RS 6301 replacements are readily available from any bearing supplier, but it is a good idea to use a quality brand given that they are on a fairly important part of your bike! I used SKF bearings.