Is it by chance a Tri-Mark Lock? They were a real problem child in that vintage.
If it is then I have some knowledge.
Has the door been opening hard and sticking, perhaps the reason the inside handle broke?
If so my advice open the door as little as possible if a all until you can replace the whole unit.
Tri-marks in that era had a tendency to break in the door closed position. The internal mechanism breaks. But before that they will open hard. If that happens they are extremely hard to replace.
However if you can get the door open they are maybe a easy 20 min job. A whole new lock should be about 80-90 bucks.
On ours we had to tear part of the door apart to replace it, not fun.
If your lock is not a tri-mark then I have no knowledge