Supposing someone has a small brain tumor and it is located in the posterior cranial fossa. Is there a good chance of survival?

Best to discuss. with your neurosurgeon who has viewed your MRI and knows exact location size etc...
Depends. It really depends on which type of tumor it is and also it vicinity to nearby structures. I presume you have seen a neuro oncologist or neurosurgeon. This would be a good conversation to have with one of them. Best of luck.