In the original game, magic was cast by first finding a special orb called magicite. This magicite would then allow a player to use a certain type of magic. Fire magicite allowing them to burn their enemies, cure magicite allowing them to heal themselves, and so on.
In Dungeons and Dragons, however, magic is tied to particular character classes, who prepare magic according to spell slots, and who may thereafter cast their magic any time, so long as slots remain.
Rather than attempt to force one system to work harmoniously with the other (as I believe the accuracy to the original setting would come at the expense of player enjoyment) I have done away with magicite as an enabler for magic. Instead, I would recommend that magicite be treated as a very rare and powerful consumable item- something that would allow spellcasting characters to regain an expended spell slot. Thus, instead of finding fire magicite, players could instead discover a piece level 1 magicite, which would, of course, recover a single spell slot of first level.