Mass flow rate would be approximately calculated by taking the size of the motor, multiplying it by the RPM that you want to focus on (i suggest making a spreadsheet to see the mass flow rate across the RPM range) and dividing by two since the engine only exhausts every other rotation (since it is a 4 stroke). These calculations assume 100% volumetric efficiency also.
Remember that you must divide this final answer by two, because you are making a dual exhaust.
