I set the engine, with the prop and spinner mounted, on the engine mount. Then try to balance the plane. Sometimes a small drop of thick ca glue is helpful in keeping the engine where you put it. Remember to account for down and right thrust angles as per the instructions. Some planes have these angles built into the firewall, some give instructions how much of each to use and if you're lucky, they will give some insight as to how to accomplish this. From there, I saw a post earlier that suggested using a tool that I think is called a dead center engine mount locater from again I think, great planes. It has a small cone with a small drill bit that protrudes from the center of it. I don't know how I got along without mine. They are not much $. If you need to re-fit a new mount to the firewall, again try a small drop of thick ca glue on each side to glue the engine to the mount temporarily. and use the tool described above. You can plug the old mounting holes with dowels glued in the holes with either epoxy, or slow ca. Cut and sand dowels flush with the firewall, then re- drill and insert blind nuts. Don't forget to use lock-tight on all bolts. I would use locknut's with the neoprene inserts if possible.