The easiest method I know of is to make a temporary clamp to hold the engine & mount together so that they can be slid into alignment. You drill a hole in the center of the firewall, making sure that it is a large enough diameter to accept something like a 6-32 bolt and also has enough play to let you move things around. Then, you pass the bolt through a washer or small wood plate, whatever will let you grab the engine mount and pull it against the firewall. On the other side (inside the fuselage and behind the firewall), you will screw that bolt into a combination of a blind nut mounted on a plywood stick. That stick is big enough to grab the firewall from behind and allow you to snug up the assembly to the firewall and still move it around for alignment. Once it's all set, mark the mounting hole locations and then remove everything for drilling.