You have all of the necessary ingredients to have pure RIR. Here is what you need to do. Let the RIR breed the Aseel pullet. when she starts laying, she will set. When she sets, reach under her and take out the eggs. Throw them away. Replace them (do this all at the same time and at night) with eggs from purebred RIR. (Although the crossbred birds would probably be more hardy and interesting).