Actually fairly easy to pull off.
Go to upgrades and create a custom upgraded based off of Long Rifles.
Go to any potential units that can get the item and apply to them "upgrades used:" the custom upgrade you just created.
Create an ability for the item. It can be a dummy ability or something you want (maybe might want to base it off an orb ability). In the Art - Missile Art, specify the new graphic for the new and improved missile.
Create the item that you want and give it the dummy ability so it changes the missile art. Write a trigger that when a hero unit acquires the item, it sets upgrade level of the custom long rifles upgrade to 1. And there's your upgraded range and missile. You may want another trigger to detect when the item is gone (and unupgrade)