Considering what I am assuming to be the age of the unit (1969 or earlier) a recommended match for the air handler would not be available. If it is really that old you would need to replace the air handler for the sake of efficiency and required ARI matching by the contractor you will hire to do the work.
Let's see what others may say and possibly correct me on. I could be wrong!
Every problem gives us the opportunity to choose from several solutions: stupidity, ignorance, indifference, or brilliance. Which one will you choose?