Hello Sophie

> Is replacing the internal transformer a difficult but documented operation

It is the opposite ;-) it is an easy but undocumented operation - for a
knowledge person

The schematics are available here
http://fa.utfs.org/diy/korg_monopoly/
or here
http://monopoly.highspies.com/manuals.html

If this information is not enough for you, then ask someone who is able
to use this information. In UK I can recommend Tony Allgood from
Oakleysound.
But any other repair shop for electric musical instruments (or radio and
TV electronics) will be able to do this. But it will be more expensive
than a stepdown transformer.

Example for Stepdown transformer see
http://www.amazon.co.uk/Step-Down-Voltage-Convertor-240V/dp/B000H9HU5C/ref=sr_1_2 ie=UTF8&s=electronics&qid=1238619815&sr=8-2


Regards, Florian