the avi monos are a great sh buy...I have been tempted on a few occasions...never took the plunge..
with the transformer...tx...there are a few routes..
1/ buy a mains voltage convertor...try maplins for a 120v output or do a search on the web...some of these building companies, too..some tools run off 120v...
2/ if the primary of the transport can be wired for 230v..it should say on it if you have a look inside...may do may not..makes it easy...take it to a good tv repair shop and they will do it on the cheap...its only a few wires...
if muse have any contacts try emailing or even phoning to get it from the horses mouth..
4/ failing all that, you will have to buy a new transformer with 230v primary and the correct secondary voltages...and ample current....and get it fitted by someone decent, again maybe a good independant, or get some quotes from audio specialists like emporium or audio laboratories leeds, ...similar....
maplin , rs and farnell....you may have to get one made...the company who supplies maplins nice toroids do that I think...