I would recommend installing a new high-efficiency boiler in lieu of replacing the firing chamber unless they can retrofit new burners, controls, and box to increase the efficiency up to today's standards but, without power venting, you're probably better off replacing the whole thing. Check to see if there are any tax credits/rebates available from your local utility or propane distributor, State or Federal tax credits (usually for Energy Star products) to help with the cost. What is the BTU rating for your boiler, do you have baseboard or radiant flooring, and how many zones in your home? The amount of windows and size, insulation, exterior wall thicknesses, all have to be taken into account to size the boiler correctly (among other variables). You might want to get a second opinion from another heating contractor and make sure the burner is adjusted properly before shelling out a few K for a new system. Hope this helps.