If you can manage the ductwork, a traditional forced air system is the way to go. If not, then "it depends". You can stick with your gas-fired boiler system or check out minisplits with multiple heads (I don't like these, except in garages/attics, as I think they are ugly).

Ductless mini split heat pumps deserve some consideration in new construction, but especially in retrofits and remodels. It's more efficient to pump refrigerant than it is to move air in a home, your get zone control, and your home's air infiltration rate won't be increased by your ductless system.

The Mass Clean Energy Center did a field study on cold weather heat pumps in both ducted and ductless installations. You can find a number of case studies here: https://www.masscec.com/blog/2021/09/13/masscec-pilot-showcases-success-whole-home-heat-pumps

You could keep your existing hydronic heating system for a season and use it as back up heat if necessary. If not, you could remove it at your convenience.

sktn77a, there are "ducted" mini split systems - some call them "ducted ductless". They work about the same as a traditional split system insofar as having air handlers (can be more than one) and small ducts that feed supply grills. There are also wall/near floor units that look like a heater or radiator unit and also ceiling ones, both of which also avoid the unsightly thingamajigs mounted high on a wall that come to mind for most people when thinking of mini split equipment. Some of the ducted ductless type are particularly efficient because as with minisplits generally, the outdoor units have a range of operating levels and temp control zoning can be accomplished even room by room.

My questions are, what is most efficient in my area? What is most reliable? What requires the most/least work? Should I stick with what I have and replace when it starts to fail or spend the money now to upgrade?

I will assume by most efficient you are asking what is least costly to operate. In Massachusetts I would think heating with natural gas would cost the least. If you post your gas and electric rates a comparison of the costs can be done. Steam heating is the most expensive to heat with natural gas. You are lucky you don't have an oil fired boiler. The one positive thing about a steam boiler it has no moving parts and is very reliable. If you you do simple maintenance and keep the water at the proper level you should get another 20 years of service out of it.

I assume you will use window AC units for cooling. If you want to eliminate them then this would be time to see if either the addition of a central forced air system or mini splits could be added as part of the renovation.

The previous owners removed all but one radiator from the second floor, we aren't sure why.

This is a big red flag for me. One radiator on the second floor is not going to keep the entire floor of a house located in Massachusetts built in 1915. Do you have the radiators? If so a simple option would be to reinstall them. If you like the idea of having no radiators then investigate the forced air and mini split options. Your general contractor is not knowledgeable about HVAC systems. You need to talk to HVAC contractors to discuss possible options.

Thanks for the input everyone. I'm trying to get someone more knowledgeable to come take a look, people are just scheduling pretty far out even for quotes. @mike_home yes we have the radiators, and will for sure reinstall them if I stick with the steam boiler.