I just use stainless bolts that are about 1 inch longer than needed to clear all the layers of deck. I then double up LARGE fender washers on the underside of the bolt and double nut all the bolts with the last nut being a nylon insert lock nut.
You should have a front access panel that will allow you to lay on the front deck and reach any of the areas the bolts will need to be. Test fit your motor to several different areas and make sure you have access to the underside of each before doing any drilling.