You could also use a dowel rod to fill the old holes. Insert a dowel rod to fit the hole and glue it in place, then saw it flush with the firewall when the glue is dry.

While it's possible to make a wood engine mount when the wood is part of the frame of the plane, you want to steer clear of that here. Unless it's constructed properly the chances are that it will fail. This could result in a dangerous situation if the motor is running as it could hit somebody and injure them. Install a regular engine mount.

Hope this helps

Ken