You can have an inventory using variables. One variable for the item, and another for how many of that item you have.
Create numbered huds for each item, and assign them a hud position from the top of the screen down. When you press the inventory display key (SCANCODEKEYPRESSED=X), if the item is in inventory (VARGREATER=0) then show that item hud with the proper number (VAREQUAL=3). Then drop down to the next position on screen and check to see if the next item is in inventory. If it is, display it. Rinse and repeat if necessary.
It could get extremely tedious though. You may want to keep the inventory down to just a few possible items. Each time an item is used, it has to be subtracted from that second variable.
That's one way.
Best.