Many of the kits offered refer to the engine kit as "firewall forward" kits. The firewall is the wall between the engine and the pilot and the engine is hung on it. So the engine is installed or mounted on the firewall, and then all the electrical and fuel connections are made to feed it.
Make sure the engine you select has proven to be compatible with the aircraft you've selected. In most cases, the aircraft designer has already done this. In cases where you're experimenting with a truly experimental aircraft you might need additional testing and documentation to assure the FAA that your new combination is safe to fly.