The forums actually already have a few open source Noise plugins you can look at for reference (or simply use and move directly to playing with Terrain generation):
Simplex Noise:
https://forums.unrealengine.com/showthread.php?94847-Plugin-Simplex-Noise-1D-2D-3D-4D-Fast-Perlin-Noise-Version
UnrealFastNoise:
https://forums.unrealengine.com/showthread.php?121155-UnrealFastNoise-Modular-Noise-Generator-Plugin