Bernd on xs400.net in Germany have found that XJR1300 conrods fit the XS crank.
He also used GPZ900 standard pistons, whilst at 72.5mm, still ok to bore out the barrel to that size.
XS .............. => XJR
1L9-11656-00 => 36Y-11656-00
1L9-11656-10 => 36Y-11656-10
1L9-11656-20 => 36Y-11656-20
1L9-11656-30 => 36Y-11656-30
The pistons need to be modified on a milling machine and new valve cut outs made for the XS valves