Measure the distance from side to side width of the frame at the front of the motor using the crank as center. Do the same at the rear using the output shaft on the trans. Once you have that width, you go to the center of that width and that is the center of the frame.
Put the motor into place and if the front is an inch off center to the right you put the rear an inch off center to the right. The center of the motor will then be even an inch off the center of the frame.