I think it is much to early to speculate on what will happen next. While the outcome at Whipsnade is quite dramatic and impactful and emotional keeper staff ... for now it seems that we probably can not hope to see a rhino calf from the remaining F Behan at Whipsnade. Probably it is best to hold off for now on any further speculation on replacements....Presumably that is why Beluki had much the more advanced tumours, given she stopped breeding a long time ago now.
I would think it would be easier to source a new young female from Europe, given there are many zoos breeding them nowadays. But importation may not be so easy given current restrictions and bureaucratic delays.
BTW to put some perspective to the reproductive tract issues with both GOHR cows at Whipsnade: F Beluki had her last calf in 2010, whereas F Behan had her last in 2015. Perhaps the condition for which both rhinos had been treated lately may correspond to the around and about timings at which both cows started failing to breed (speculative though ... this may be, but perhaps the cysts and reproductive tract was affected after the last births .... for both cow rhinos).

