Replacing Convector, upright heaters Choices?

I have a 1950 home that installed hydronic upright heaters throughout the house. (Also known as kick space or convectors.) The motors have since gummed up. They incorporate a squirrel cage and motor with a variable rheostat. The heaters make an extreme amount of noise, which turn on way too early blowing cold air before the pipes come up to correct temp, except the few that I replaced, which have a small aqua-stat and 2 speed on/off.
However I was persuaded to install smaller than adequate sizes in the few I have upgraded. I have made efforts to locate original replacement parts but the company no longer exists. I would have to go with Turbonics or Beekman Morris, and perhaps a few other choices. It’s a shame because the small radiators that exist are all in good shape (I believe?) and are standalone units separate from the motor, fan, and rheostat assembly.
To give you an idea of our layout, Its 2 story ranch (with each its own zone) with lots of large windows, vaulted ceiling and don’t have an incredible amount of base wall space.
We have heard all types of different advice in trying to get a better system. Everything from installing a whole new forced hot air system. This would involve in addition some super major construction work! Just replace exactly what you have with all new units which would cost almost 7-9k+ just in parts alone (I seem to think a couple of larger commercial units would be a large cost saver on this route) plus cost of labor. I was advised to replace everything with baseboard all over. I thought the plumbing for baseboard and Electro-Hydronic was different (feeds and speeds as told by some?) I also have been advised that temperatures for baseboard and hydronic vary enough to make a difference, so many variables. Who to believe? So I am at square “A”
I also did some research for replacing the individual parts in each unit. The radiators, case and covers are good so I would need new motor and squirrel cage with an aqua stat, and some sort of switch all to fit in tight existing space. I did not have much luck in locating a motor or squirrel blower cage! (In correct sizes) I did find some motors that cost as much as a full replacement unit assy. I have also tried substituting some muffin fans and also some individual slightly bigger fans but without much success! (As far as speed and heat output)
So I would really like to hear from some experts’ suggestions and solid advice on how to get some serious heat in this house. In the meantime I am currently working on replacing new insulation along with chipping away replacing modern windows which should also contribute to better insulating the home as time allows it. Please excuse me if I misused some terms. I know I may have left out plenty of details. I will try and answer questions best I can.