Actually the chances of motoros breeding is quite high, they are probably the easiest ray species to breed in captivity.
Females should be sexually mature when they reach about 12 to 15 inches in diameter, males probably a bit smaller but you can see when males are ready to breed by looking at their claspers, when they become fleshy and rolled the fish is ready for mating.
Always try to get a male that is a bit smaller than the female, mating can get a bit rough and a smaller female could get badly injured or killed by a larger male, a seperate tank or a divider is usefull to give the female a bit of space if the male gets a bit to full on and she isnt ready.