You need to modify the connections to the two transformers as shown in the dotted box, "230 VAC OPERATION PRIMARY WINDING".  

From the ON/OFF switch to the 115VAC winding of the top transformer as drawn.

Then the wire leaving the COMMON terminal of the top transformer is disconnected from the other wires it is connected to and label it "1", as an example.  

Also disconnect the wire from the 115V AC terminal of the lower transformer and label that wire "2".  

Now connect wire "1" (which is only connected to the common terminal of the upper transformer) to the 115V terminal of the lower transformer.

If possible remove the wire labelled "2" completely without disturbing any other connections.  Or install a insulated wire terminal (blue point connector or similar) to avoid a live wire in your cabinet.

Doing this effectively puts the two transformer primary windings in a series circuit, allowing 240V (230) AC operation.

DON'T FORGET TO HAVE THE GAME SWITCHED OFF AND UNPLUGGED BEFORE DOING THIS CONVERSION.