Ouch! Sorry to hear that. And that is why you ALWAYS should keep multiple save files, especially for games like these with lots of missable stuff. Cheat Engine is more for boosting the amount of things you currently have. So, it is very useful for completing the 10,000 battles quest, to max out your GP, to instantly get your characters to level 50, and in LP3, to quickly max out their Slayer Points. I don't know if it can be used to hack a specific item in, though!
Sorry, but I really don't know of any other way! But you can't quite get ALL of the stuff in the game, anyway. When you beat Tamara, the witch at the Upper West Plains, you are forced to choose between 2 unique items, either the Tamara Ring, or the Calmness Robe, and you can't fight her again to get the other one! I'm also pretty sure that you can't get the Goldeen Bow, either. If you can, noone has ever found it, or posted where to find it!