That 35% Giles is quite a bit larger than the typical 50cc ARF produced today. I would go with a larger engine but would favor a twin for less vibration and it should easily fit in the huge cowl. I would also put the airframe on a diet to get rid of some of the excess weight. If you have a finished weight in the mid 20 Lb. range, it will fly quite well.
The Giles flies very well but it was not my favorite of the Lanier planes. I much preferred the Extra, Laser or Staudacher (sp?) all of which I still have in my hanger.