Somewhere between posting first time and creating github i tried to change modname\modId from arcanapp\arcpp to master_of_magic\mom, and it seems that there was some unchanged parts left ) ok, i fixed that. However, i thought that folder in /assets/ is not necessary named as modId, as otherwise how did it found en_us.json in /lang/? Name of item was shown correctly.
Anyway, that solved problem only partially. After fixing name of folder in assets, model started to register, but texture
[03:07:52] [Server-Worker-4/ERROR] [minecraft/AtlasTexture]: Using missing texture, unable to load mom:textures/items/sun_crystal.png : java.io.FileNotFoundException: mom:textures/items/sun_crystal.png
is still missing, for unclear reasons