I've figured out an issue with items and I want to fix it to myself and provide a bugfix for you, but I cannot compile it or I don't know how to compile it correctly. I am using the default mdk for 31.1.0 of forge and integrated the code inside of the src/java/main folder with adjusting the dependencies, but it does not work. The build process crashes with:

[30Apr2020 00:00:39.190] [modloading-worker-1/INFO] [net.minecraftforge.common.MinecraftForge/FORGE]: MinecraftForge v31.1.0 Initialized [30Apr2020 00:00:40.795] [Render thread/ERROR] [net.minecraftforge.fml.javafmlmod.FMLModContainer/]: Exception caught during firing event: net.minecraft.block.Block$Properties.notSolid()Lnet/minecraft/block/Block$Properties; Index: 2 Listeners: 0: NORMAL 1: ASM: class de.ellpeck.prettypipes.Registry registerItems(Lnet/minecraftforge/event/RegistryEvent$Register;)V 2: ASM: class de.ellpeck.prettypipes.Registry registerBlocks(Lnet/minecraftforge/event/RegistryEvent$Register;)V 3: ASM: class de.ellpeck.prettypipes.Registry registerTiles(Lnet/minecraftforge/event/RegistryEvent$Register;)V 4: ASM: class de.ellpeck.prettypipes.Registry registerEntities(Lnet/minecraftforge/event/RegistryEvent$Register;)V 5: ASM: class de.ellpeck.prettypipes.Registry registerContainers(Lnet/minecraftforge/event/RegistryEvent$Register;)V java.lang.NoSuchMethodError: net.minecraft.block.Block$Properties.notSolid()Lnet/minecraft/block/Block$Properties; at de.ellpeck.prettypipes.pipe.PipeBlock.<init>(PipeBlock.java:60) at de.ellpeck.prettypipes.Registry.registerBlocks(Registry.java:88) at net.minecraftforge.eventbus.ASMEventHandler_2_Registry_registerBlocks_Register.invoke(.dynamic) at net.minecraftforge.eventbus.ASMEventHandler.invoke(ASMEventHandler.java:80) at net.minecraftforge.eventbus.EventBus.post(EventBus.java:258) at net.minecraftforge.fml.javafmlmod.FMLModContainer.fireEvent(FMLModContainer.java:106) at java.util.function.Consumer.lambda$andThen$0(Consumer.java:65) at java.util.function.Consumer.lambda$andThen$0(Consumer.java:65) at net.minecraftforge.fml.ModContainer.transitionState(ModContainer.java:112) at net.minecraftforge.fml.ModList.lambda$dispatchSynchronousEvent$5(ModList.java:125) at java.util.ArrayList.forEach(ArrayList.java:1257) at net.minecraftforge.fml.ModList.dispatchSynchronousEvent(ModList.java:125) at net.minecraftforge.fml.ModList.lambda$static$1(ModList.java:96) at net.minecraftforge.fml.LifecycleEventProvider.dispatch(LifecycleEventProvider.java:71) at net.minecraftforge.fml.ModLoader.dispatchAndHandleError(ModLoader.java:197) at net.minecraftforge.fml.ModLoader.lambda$gatherAndInitializeMods$24(ModLoader.java:189) at net.minecraftforge.registries.GameData.fireRegistryEvents(GameData.java:969) at net.minecraftforge.fml.ModLoader.gatherAndInitializeMods(ModLoader.java:189) at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$2(ClientModLoader.java:97) at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$5(ClientModLoader.java:113) at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:97) at net.minecraft.client.Minecraft.<init>(Minecraft.java:393) at net.minecraft.client.main.Main.main(Main.java:141) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:102)

You don't need to use "the default MDK", all gradle files are already in this repository. You build the mod like any other. Look up a tutorial online.