Why two heating systems?

Electric heat is dirt cheap to install but usually very expensive to run unless the house is super insulated.

Do a heat loss of the house and see if the existing boiler has the capacity to heat the whole house, if it does extend the hot water system to the second floor as a second zone.

A cheap improperly designed hot air system is not going to be comfortable or economic. A properly designed hot air system will cost a lot more than extending the hot water system.

Bob

what about cooling? Maybe add an air handler with a hot water coil for the upstairs.

A load calc for the entire home first. then see if the boiler has enough capacity to run the up and down.

1964 was an era of bigger is better. Chances are the boiler of that time is generously oversized for the house and would probably heat both floors. For some reason the system was not extended to the 2nd floor. May it wasn't finished. Are or were there bedrooms on the first floor? Some people didn't believe in heating bedrooms then, ( I know, water would freeze in the room).
Is your 4000 square feet actually 2 floors times 2000 each?

EBEBRAT-ED has brought up an interesting subject concerning pure aluminum romex wiring, something to be avoided or the consideration to be dealt with financially. At that time there was also copper-clad aluminum romex which looks to be copper but if you scrap it the aluminum shows up; it is also one gauge size larger than copper was. (#10 AWG equals #12 copper for 20 amps. ie kitchen ckts.) Special connections are needed at each box (outlet-switch-light) for safe operation. Thankfully this pure aluminum wiring mostly showed up in trailer houses of that era. (Don't get me started!!) A home inspection by removing the main breaker panel cover will tell you a lot.

BobC said:

If aluminum wire terminations is done right your ok, if not the danger of a house fire is very high. The problem is a copper aluminum interface will oxidize and that raises the resistance and that creates heat - a very bad combination.

Bob

It doesn't just oxidize -- the aluminium also expands much more than either copper or steel when it gets even a little bit warm, and this loosens the connection -- just a little -- which then gets even warmer, which loosens... you can see where this is going. There are special devices -- switches, plugs, fixtures -- to use with AL or AL/CU wire. They don't have this problem, but they are hard to get. If one is going to replace a fixture in such a house, be absolutely sure that the replacement fixture is correct!

Depending upon where the OP is located?? Some local codes did not allow AL or AL/CU wiring at all. Hoping we don't scare them away from the house until the facts are actually known. We just hope to have them well informed of any potential minefields.