The TR1 2nd oversize piston part number is 5A8-11636-00
the 1990-92 XV1100 1st oversize piston part number is ITA-11636-00
different prefix, but same number - good chance I think that it's the same piston.
£79.16 each... ouch! ring set £37.41 each... ow!
but, xv1100 pistons are still available
gonna give em a try, i'll post results
Bob