First of all welcome to the hobby but make sure you enter from the main door.

The project you undertook to finish is definitely not the right way to learn flying. Flying is a different story from building.

Start with a trainer, find somebody at a club to teach you flying and when you feel ready try your grandfather's scale plane.

Remember you need many flights on your own in order to be ready for a scale plane.

As far as engine size I think you will need 0,30- 0.40 cubic inches size.

Stelios