I've got the plumbing for my old baseboard hydronic system (circa 1963 +/-), but the boiler is gone. Don't recall it's btu rating. My guess would be in the 125,000 BTU/hr. range. I've also got a heat pump for all HVAC.

Should I get a small natural gas boiler and heat (supplement, at least) using this system ? I remember how nice the warmth was from baseboard heat.

Should I abandon in-place, and live with the baseboard equipment ? Should I remove the whole hydronics system and patch the holes in the floor ?

Ideas and comments/suggestions appreciated.

You are the best one to answer your own comfort vs budget question.
Perhaps if you get a quote on a boiler it will help you with the decision.
Don't know if you have a geo or air source heat pump, but one thing that comes to mind is a boiler would require a much smaller generator than a heat pump. Depending on your heat pump design, gas auxiliary heat may be cheaper if you are currently using electricity.

Another thing to consider depending on where you live and your level of interest is a wood boiler.
j

A lot of 125KBTU boilers installed in 1963 (or even 1983) were in houses with real heat loads of under 50KBTU/hr. What's the heating-mode output rating of your heat pump?

Fin-tube, or cast-iron baseboard?

Tepid-air heat pumps aren't always big on comfort if the ducts are blowing on humans, creating wind-chill drafts, etc. Those with variable speed air handlers & compressors can be pretty efficient AND comfortable if you stick with a single-setpoint strategy rather than overnight setbacks (where they have to work harder, at lower efficiency blowing more air to get the place back up to temp.)

Have a professional perform a heat load analysis and replace the boiler. As you have noted, there is no substitute for hydronic heating. People from the east - where hydronics is more common - come here to Minneapolis and want us to take "this air blower thing out and give us radiators". Your contractor should measure the house, give you the load and measure the radiation as well.

What MA (BadgerBoilerMN) said!