Ok, you have a few options.
Replace the coil with a special order made to fit from Lennox. The original coil has been deemed obsolete for sometime now. Labor - $500, coil, $1,100+, r22 refrigerant $400 = Aprox $2,000
Order a generic coil that is coil to fit and modify. Labor, refrigerant, coil = $1,600
However I would strongly recommend not t invest that much money in a 30 yr old air hander. Especially when you buy a entire new air handler for the same amount of money SEE HERE
Air handler - $700, labor $1,000, refrigerant $400 = $2,100.
Now lets say I am low on a new unit and I add another 50% (1,000.00)
We are looking at $3,000.00......
So the question is...if you have a car that needs $2,100.00 of repair...or they offer you a brand new car for $3,000.00, which is the best deal?
After $2,100 in repairs you still have an old car...
My opinion is to not invest any money in a 30 yr old unit.