I guess the North Americans dont take much notice of these differences as apart from the 1980 CT110 they all have 2 speed transfer cases with bikes up to 1986 imported and they dont have any 12V bikes. Their main choice then is 90 or 110 and what year.

After watching the trouble they had getting up very steep hills in the "Hard Core Postie" Snowy Mountains ride and also Nathan Milward with the same problem on his ride from Sydney to London and as a CT90 owner I wouldn't have one without a 2 speed. Therefore we only have the choice of CT90s which are fairy uncommon in general here compared with 110s. The other option is CT110AGs most of which aren't road legal and are all 6 volt. They have been street legal for the last few years. I'm not sure about all the differnent suffixes in 110s.
The dream of a 12 volt late model CT110 with 2 speed box is really just that without major engineering.

While I dont have 1st hand verification the following I do watch various forums with respect to this and can contribute the following.

The CT90 and 6 volt (pre 1998) CT110 posties have different engine cases so you cant just add a 2 speed box. The cylinders are also different so you cant add a 110 cylinder to a 90 case. There is also a defference between pre and post 74 (I think?) CT90 cylinders. The CT90 has a magneto spinning in coils while the 6V 110 has a coil spinning in magnets. The cases are therefore also different between a CT90 and a CT110AG or North American one. The way the electrics are set up is also different so if you were to put a 110 motor in a 90 frame you should also swap harnesses.
The 12 volt posties (post 98) obviously have a 12 volt alternator which is quite different to the earlier ones and I'm pretty sure the case is different.
There are some other difference between the earlier and later model 110 Posties such as thicker spokes, different forks with internal springs and improved seating position. Forum member Bernard from NZ (Platypus 121) has posted about these previously.
To summarise engine case/ stator housiings casings there are CT90, 80-98 110 posties, post 1998 110 posties and CT110AGs.
From what I can tell all the frames are the same from the later model 90s to all 110s.
 It's fairly easy to find road compliant frames so if you bought on older non-road compliant AG you could swap frames and I have seen bikes to which this has been done on eBay and am planning to do the same if I can find a cheap AG. You can even buy a brand new road compliant one from your Honda dealer for about $4000 but be aware that they are still 6 volt.
The CMSNL site only goes to 1993 I think so doesnt cover the 12V posties. It seems no-one has every been able to find a comprehensive parts fiche for these. You just have to rely on the Honda dealers or ONETEN Motorcycles in Caboolture.

I'm sure that if any of the above is incorrect someone will correct it.
Hope this helps.

Justin