Hi iso2013! Welcome and good luck!
Most of the work of updating to 1.9 is already done, but the part that I'm stuck with is the registration of biomes. We need some way to register all the custom biomes in a world, and unregister them when the world unloads. On Spigot, we can assume that the world is never unloaded, but we can't do that on singleplayer Forge.
Back in Minecraft 1.8, the server (local or remote) would send information about the biomes, and the TerrainControl forge mod would put that information in the biome array in the BiomeGenBase class. Now that array is gone, and a new registry takes its place. Forge has modified this registry somehow, and Minecraft seems to ignore all our custom biomes.
The instructions for setting up the workspace are located over here, but you already seem to have figured out most of it. The remaining of the instructions are about how to get the Play button working in IntelliJ.
If anything is unclear, we'd like to hear it - then we can improve the documentation.