The only times I recommend taking out hot water heat of any kind and replacing it with forced air is if the house froze up and everything is destroyed or if it was installed so porely that it can't be made to work right no matter what.
In most cases you will not feel as comfortable with forced air verses hydronic heating.
There are a lot of options to air conditioning hydronic heated houses today. There are also many high efficient options for a new boiler and controls that will greatly improve the efficiency of your system over a 50 year old boiler.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.