ORIGINAL: Charlie P.
The center of gravity is shown on the plans or described in the manual - i.e. 3-1/2" from the wing's leading edge at 12" from fuselage's centerline on either side.
Doesn't matter if you've added a Chevy 350 engine or taped a duck to the nose, the center of gravity will still be there once you've compensated with sufficient tail weight.
However, if you HAVE put a duck in the nose, you're not going to be adding tail weight. Unless it's to the duck's tail.
And if you do THAT, then don't tape or glue it to the feathers. They will not hold over an ounce each before pulling out. Plus the duck will be pissed off.
Choose a goose instead. They're heavier to begin with and if you have to attach the weight to those feathers, they hold better.