I've never had one and have mounted a good number of engines. Yes, the tool will help you avoid mistakes and will make things easier, but modelers were mounting engines before the dead center tool came along too.

As for the hole size, a lot of that depends on your mount and how the front end of the plane is made. You can drill and tap the plastic, or you can make the holes a slip fit for your screws and put a nut on the bottom. Use loctite or a a nut with the rubber insert to make sure they don't vibrate loose if you go the nut route.