Depends which quest rewards you mean. Most (if not all) quest items and some rewards you can re-obtain from the appropriate quest npc, or by simply backtracking a bit. The whistles for the Holy Grail quest spawn post-quest regardless of whether you have the tablecloth.
Silverlight you can re-obtain from Sir Siegfried Erkle at the top floor of the Legends guild (he also sells Dusty keys and Maze keys).
If you somehow don't have the watchtower spell unlocked despite having finished the quest, a senior mod can help you with that. They can also help with any other quest-related issues. (some rewards like the amulet of accuracy you can also just trade from an alt or buy from other players).
Best of luck, and welcome to RSCRevolution!