Does that beast still have the origional burner in it? It is a lovely example of an old school cabinet boiler. Efficiency is probably in the low 70's with the original burner. If you have a newer retention head burner, somewhere closer to 80% Because it is a high mass boiler, system efficiency is pretty low, but believe it or not, not all that far off new steam boiler efficiency. Steam is a pretty poor use of your energy dollar because you need to get the water to 212 before anything happens. That said, you can indeed pipe an indirect off the system. forget about the old tankless coil, you will have to tap a feed and return below the water line of the boiler. Usually there are tappings somewhere that can be used for that purpose but getting the plugs out may be a challenge. Of course, by the time you buy the indirect, a circulator, valves, an aquastat and all the piping it will cost about 6 times what the original boiler cost. Better off with either an oil fired water heater, gas or even an electric for the cost. If you have the cash though I would seriously look into converting the system to forced hot water and changing out the boiler. In the end the conversion will probably pay itself back in fuel savings in under 10 years.