Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

To make it easy and keep costs down, you could consider using copper tube for your connection.
Brake tubing is available in 3/16", relatively thickwalled, and therefore no tube bender is needed, as you can bend it (within reason) to fit your setup by hand, without flattening or creasing.
Any auto-shop would have a small length of that cheap as chips.
You will need fittings soldered in at each end, to mount it on the boiler and engine, but getting hold of those would need a precise knowledge of which threads you have in each end.

I don´t have a 504 boiler, but it´s a relatively large boiler, and if fired to just chuff away gently, i think you´d start being full, before the boiler is out of water.
A feedwater pump is a neat item, but rarely needed for occasional runs. Doesn´t take that long to kill the heat, let the steam out and refill the boiler for another run.

When it comes to firing it is relatively straight forward, as long as you use a fitting meths/alcohol or gasburner.
Start by inspecting the safety valve, to ensure it isn´t stuck. Use a small funnel to fill the boiler approx. 3/4 full with demineralised water, and light the fire under it.
If you notice any leaks during the process, just kill the fire and fix the leak by tightening connections slightly or replacing gaskets, whichever does the trick.
If the safety valve start lifting during your run it´s all good, but you clearly have to much heat for you current steam needs, and might want to attend that, either by cranking up the engine, or reducing the heat.
(Do keep an eye on the pressure gauge, if the pressure keeps climbing with the safety valve blowing, the burner is oversized for your boiler and unsafe to use under this vessel.)

One thing of strictest importance though...
NEVER EVER leave the boiler unattended when there´s a lit burner under it, for any reason whatsoever!!!
If the phone calls, or you want that quickly made sandwich, kill the fire before you remove your attention!!!