I am so sorry to hear about your Golden retriever. My name is Rebecca. I am a veterinarian and will do my best to help.
The reason for doing an MRI is to determine if the tumor can be removed surgically, although that would be expensive with no guarantee of removing it all safely and completely. It would show how big it is, where it is, etc.
The prognosis is likely not good. If it keeps growing into his brain he could have more frequent seizures, he could go blind, and it could eventually kill him.
Your choices are not great: 1. Start anti seizure medication like Keppra or Zonisamide or Phenobarb which might be better for seizure control than just Valium.
2. Do the MRI to find out the extent and location of the tumor, to see if there is any chance of surgery helping.
3. Consult a veterinary oncologist to see if there is any chemotherapy that might help.
If he were mine, I would probably go with seeing an oncologist to find out all my options. A board certified veterinary oncologist would know the latest treatments and protocols, and having a consult with one would be where I would start if he were my own dog.
Let me know if this helps and what else I can answer or help with.