Here are a few special items ripe for conversion. I'm sure some of them have been done (maybe even in the RPG itself--I don't have it handy to check), and I'm not the one to do them, but perhaps someone else will pick these up.
Psychic Ring: Found in the Cauldron of Fear in Zaryx, seems to aid in psychic combat.
Grey Crystal Ring: Found in the Jungle of Horrors in a magic item shop, and functions much the same as the Psychic Ring.
Lodestone: Other than being particularly heavy, it doesn't seem to do much, though a lodestone traditionally has magnetic properties. In any case, it's magic, and you find it after fighting the Vordaks in the Jungle of Horrors.
Sabito Root: Not a Special Item, but cool all the same. The “blue pills” allow you to breathe oxygen underwater through your pores. You can find them in the Cauldron of Fear, but they’re also in Castle Death--you just end up losing them in the end.
Deathstaff: The eeeeevil Deathstaff, legendary in the Grand Master series, and very powerful.
Jewelled Mace: Mentioned already, the mace from Shadow on the Sand helps to fight against illusions at least once in the series.
Amulet of the Shianti: One of the items you can take with you from Grey Star the Wizard, it adds Willpower, and has other uses as a focus for magic.