If you don't mind I'll risk an opinion. The weak shooting performance and the rattling sounds to me as if some system vital part functions improperly. These air guns are not too difficult to attend if you are so inclined. No rocket science involved. Blow-up diagrams can be readily found, and the parts are also available from a good internet source of your choice.
Before any new spring attempts, perhaps a look inside and fix the gun yourself?
I have a vintage Diana 35 that I am restoring. It is tempting to install a new powerful spring to let it shoot like modern day high-powered guns, but in my opinion and experience, these old guns were not made for the super exceptional performance. Give them the proper attention, with the parts originally prescribed, and they shoot better than "satisfactory". My old 35 rocks! A Model 27 is also a sound performer.