Sick and weak stray cat,shoudl i call the SPCA?i don't want it to be put to sleep?

Hi - you are so kind to be so concerned, as I would be too! Although there is a possibility the cat may be put to sleep, you will be doing the right thing by calling the RSPCA. It sounds like the cat may be in a fair bit of pain, and it really doesn't deserve to suffer. Otherwise, unfortunately it may die a slow and painful death.