Been looking for affordable larger valves and aside from link (1 mm larger) haven't had luck finding the tulip sizes for the ZL900/1000.  The shipping from the UK for the Abuso valves will undoubtedly be a month.
https://www.abusoracing.com/product-page/kawasaki-gpz900-r-ninja-900-oversized-intake-valves-30mm
Found a 1988 ZX10 head but they are too large.