Since the boiler's internal controls are set up for steam, converting it to something that would work for water would be a serious hack. Depending on what your system's temp & peak BTU/hr requirements are, there is a good chance you'd be able to drop in a $200 electric hot water heater rather than an electric boiler.
What was the output temp you had set up with the propane tankless?
Was it pumped-direct or primary/secondary?
How much propane did you go through in a mid-winter month? (A rough measure of the peak BTU requirements.)