Lasiorhinus,
I don't know first hand but i have heard that for person to go to Japan and convert their licence by themselves is quite difficult. I can attest to the fact that you will find it quite difficult to learn the Law without directed guidance. You will definitely need an instructor to help and i have no idea where you could find one and one who speeks english for that matter.
As for hiring a/c i have also heard it is mighty expensive, so come cashed up.
Japan's civil aviation is geared towards air transport, their law is very air transport oriented and there are lot of grey areas when it comes to CPL and PPL theory. There is not a very big GA community here as there is not much space for airports and being so well served by burners no one really travels by lighties - so a/c are hard to come by. Having said that you might be able to find one but do a lot of research first, it could save you $$$ in the end.