Which boat are you talking about. The new daysailor? I would use stainless steel nuts and bolts. Use of large washers on the inside for backing would suffice but that depends on the size engine. So if it is a for example 2 hp Honda, that would be fine but if adding a 3 1/2, or 5 hp engine, you would need to use something larger than washers inside to distribute the load. Also, if you cannot get to the backside of the interior of the transom, then an inspection port is in order.
