Radiators

You haven't mentioned if the system is hot water (hydronic) or steam. A steam radiator can be removed and capped. Except for the persuasion sometimes required (bigger wrenches) it is simply removed and capped (never plugged). Depending on the age of a hydronic system, the water piping may be galvanized, brass, copper. Because hot water runs are sometimes run in series vs parallel, it may be necessary to plumb past the radiator. Because you say the unit hasn't functioned I'm guessing that is not the case. While it is always preferable to use a licensed plumber, this is so small a job, that it may be difficult to interest them for the minimal expense, under $200.00, unless a pipe splits, which really shouldn't happen. Good luck

Radiator is gone.....

I'm not one of the contractors you called, but I can tell you the Polaris water heater is not approved for use for space heating in New York City. All it can do legally is make your hot water. I'm not a big fan of radiant under hardwood without emission plates of some kind, but it does add considerable cost to the installation. The best way to go is with Warmboard as your subfloor and lay your finish materials over that.

Are those steam or hot water?