A 160/60-18 rear tyre is officially recommended by Bridgestone in Germany as a possible replacement of the original 140/70-18 BT54.
Note the 160 is a /60(per cent of the width) whilst the 140 is a /70 so the outer diameter will be similar.
They have two different versions of sport touring tyres (brand new Battlax T30 and few years old BT-023 model) which can also be combined.
See the pdf here, sorry the text is in German but the basics (standard rims and tyre sizes) are clear anyway:
http://moto.bridgestone.de/~/media/Files..._SC65.ashx