I have the SE ARF with a ST .51. There are a couple of ways to mount it. First I think mine came with screws that took a flat screw driver. Throw those away. I prefer to drill threw the mount and nut and bolt the engine to it. If you would rather not do that then go to Mike's and buy the Great Planes screws that have a socket head. When using a ball driver on the side with the exhaust flange you will be able to tighten them up. I also use a dead center tool to start the hole in the mount. Then remove the engine and drill a pilot hole. The engine can be thin CA'd to the mount during some of this. The CA will help hold it in place while you are marking all the holes. Then the mounts can be popped off the the engine.
ALSO I used the engine to set the CG on mine. The ARF version will not allow you to put the RX and batt vary far back. SO I mounted everything but the engine. The I put the plane on the CG machine and set the engine(with spinner and prop) on the bottom of the mount. Once I balanced it I marked the mount where the front of the mounting lug hit. Then I removed the mount and did the above to mount it to the mount. (I hope that made sense).

David