For weight analysis, sure do the summation of point masses in a spreadsheet. Inertia, rig the tri-filer pendulum. Engine and transmission assembly is not a point mass and it alone could account for 1/2 the total inertia of the whole car. Plus, it's inertia ellipsoid's axes probably won't line up nicely with the car coordinates you choose. There are some other ways to get these values, including methods for obtaining unsprung inertias.