Hi! I have create a program that will let you generate the JSon files for any purpose you can imagine.
This program is based on "generators", or user-created packages that tell the generator how to do it's job. For example, the basic item generator was already created by me. All you have to do is install it and open the program! It will ask for the modid and the item name, and when you click generate, it will generate your .json file!
It is also extremely easy to create your own custom generators. For example, you could create a generate that fills in your modid already if you get tired of typing it it. More information on creation HERE
Installing generators is extremely easy too! It's just like installing a mod for Minecraft Forge. Here's the wiki page on that.
And, like I said, I already create the basic item generator, so you can download that and start creating item model json files after installing it. Download HERE.
Downloads are all available here: GitHub Page(Including the generator creator).
Check it out, and if you think it needs something, don't hesitate to tell me!
Fell free to upload and distribute generators that you've created.