I learned that my oldest cat may have cancer

I lived through the cancer of my very favourite Boxer the year before last, he finally went, with great dignity on Christmas eve.
I still think of him each day, and my thoughts are with you: Take the advice of the vet whether the cat is in any pain, enjoy every day whilst you still have the chance and then look back on the enjoyment you and the cat had.
There are some very advanced pet cancer centres, in Cambridge in this country, for instance. Your vet should be able to advise if it is worth trying - but may not volunteer, unless you indicate that you might be prepared to pay the price.
Again, I feel for you: Men and cars I can't advise you on.