There is a mix of the mass. The mass used in the equation should include the mass of all reciprocating components (piston, rings, and if employed, piston rod and cross-head) and a portion (estimated at 2/3) of the mass of the connecting rod. The inertia forces of the crankshaft should be...
According to "Internal Combustion Engines Applied Thermosciences" the equation you reference includes a series expansion approxmation [(1 - E) ^(1/2) is approxmately (1 - E/e)] to simplify the estimation of piston displacement as a function of crank angle.
The equation I have for force is...