Hi Graeme
I too had this issue as the 125G I picked up had a bastardised air filter/cage. I have since found a 1980 175G full air box from the USA & fitter that, so all good now.
As for the elements, again, same issue. I have a couple of the red/black Uni NU2247ST which fit well but also purchased the yellow/green NU 2245ST & its way too bulky to use in the air box, so will not get another one of them. The outer section is the problem so I am going to use the inner first layer only & make another outer section as per the 2247ST.