if my catalytic converter is restricted (enough to reduce my fuel mileage by 20%) should the check engine light be on, or not? (it is not)

I bought the car with a dead miss, and I am trying to make it run as good as
possible until I can afford to have a engine put in it, I was getting 31 mpg on
premium at first, but the last two tanks have averaged 24 mpg, and it is
running doggier than it was, that is why I suspect the catalytic converter, it
has 138,000 miles on it, the distributor cap contacts (aluminum) were
corroded (I cleaned them up) but, I haven't driven it yet