The bad news is that any attempt to correct the problem you have is going to be expensive. There are many things we've learned to do while building homes that we didn't do 80 years ago. For instance, vapor barriers are common in homes built in the '80's and later, but not before. Ventilation started to be taken seriously in the 70's. We have many more options from an insulating point of view, than we had previously. So to bring your 80+ year old home into modern practice compliance will require considerable expertise and money. We've done this for a number of clients, and it is never simple or straightforward.
My suggestion would be to find a good general contractor, and have them give you an estimate. This contractor will have to look at the structure, the possible sources of moisture, the space available for installing new insulation and ventilation, and see if it is feasible.
You are right in saying that if you upgrade the ventilation you could see a lot of heating dollars escape, UNLESS you also upgrade the insulation. So you have to think of your roof and attic as a system where every component plays a critical role, and each element has an effect on the others. Get some professional help for this project, it'll be money well invested. At the end of this exercise, you can then decide whether your home is worth enough to invest the sums of money you'll need.