Code:
item script CustomPotion{
	void run(int refil,int counter){
		if(counter == 0){
			Link->MP += refil;
			if(Link->MP > Link->MaxMP) Link->MP = Link->MaxMP;
		}else{
			Link->HP += refil;
			if(Link->HP > Link->MaxHP) Link->HP = Link->MaxHP;
		}
	}
}
D1: Amount to refil
D2: 1 to refil hearts, 0 to refil magic

To take the item away after useage, there's a little checkbox somewhere in the item editor.