My current "vapor-ware" make-up water feed pump (MUWFP?) is a pressure washer pump that runs constantly, and water is either added to the boiler-tubes or bypassed by operating a 3-way valve (through a check valve, of course). The crankshaft runs the PMA and also the MUWFP.

The simplest way to start out is a total-loss system, where cold water is added and converted to steam, and then the exhaust is vented to the air. (side note: if the hot exhaust is piped to the top of an elevated aluminum radiator that is above a roof-cistern, most of it will condense into distilled hot water with a gravity feed to the cabin)

If a large insulated tank (salvaged 500 gal propane?) is kept as a reservoir, once it is warmed by the condensing exhaust, you will be feeding the boiler 200F water instead of 50 F water.

The 400 gallons of hot water can also be used as a DHW pre-heater, or for a radiant floor heating system.

The plan will probably change (if I ever actually do something) but I'd like to try 10 ft of 1/2" (dash-eight, meaning 8/16ths) stainless steel hydraulic tubing filled with sand to prevent kinking and wrapped around a pipe to form a coil. The constant duty pressure rating is 3,000 PSI and burst is 4,500 PSI.

I figure the Franklin stove and boiler tube (also the feed control valve) will be inside the shop, and the relief valve and the rest will be outside surrounded by a solid  wall and an open top.