a very primitive way would be with values in the player.
Just make a script that constantly checks for these, like lets say i'm checking for "Sti########################set to 0, that means i dont have a stick.
Lets say then an event triggers the value, so now sticks value is 1, i now have a stick. If its 2, i have 2 sticks.
Now make a gui become enabled upon hitting U, and have the gui constantly check the values for any changes
if the value becomes 1, a sub gui thing showing the item icon would pop up (you gotta make the gui icons and hide them first, obviously) so now you can see a stick in the inventory. If the value for stick is 2, #### # ##### in that same slot, but with a X2 or something on it.
{Sorry if this doesn't make alot of sense}
this wouldnt be as good though because of the limitations like, items could only appear in certain slots because that's where the invisible item icon was originally, but i guess you could check for other item icons and if none exist change the location of the gui via script |