First establish a source for replacement parts, otherwise there is no point to mess with the air rifle.
Also by your description, I am not sure if the main spring is fully cocked or not at this point ?
If you are not sure about it continue cautiously.
Second (if you are mechanically inclined) start to take the rifle apart.
Remember that the danger is in the in the main powered spring.
Taking screws out is fairly safe, pins on the other hand should be done cautiously, they usually hold back some sort of force.
Pushing out any pins that hold assemblies together should be done with a smaller diameter pin than the one you attempting to push out. The pusher pin should go in loose, but if spring tension is present it will stick and keep things from exploding on you.
If the push pin gets stuck you will know there is spring tension that needs to be released.
Putting pressure on the right component or spot will free your pusher pin and allow you to release compression in a controlled and safe manor.
At this point you can just identify the parts that look broken or excessively worn and replace them.
Good Luck