You need (and should) use an access transformer for accessing private fields. If you can join #spongedev on our IRC for a moment I can help you to set them up.
Shortly said:
-
You need to create a AT config similar to https://github.com/SpongePowered/SpongeCommon/blob/master/src/main/resources/META-INF/common_at.cfg in the
META-INFdirectory in your resource, add the fields there aspublic. -
Configure ForgeGradle to apply them (e.g. add
atSource sourceSets.mainin the minecraft block of yourbuild.gradle), runsetupDecompWorkspaceagain. -
Add a
FMLATmanifest entry with the file name of your access transformer.