steam pressure regulator

At home I have a picture and description of a very cute gas regulator for a Linotype machine. It held the temperature of the typemetal pot at a constant temperature and was only 3 or 4 parts.
I'll try to scan it you if you like.
eng> I built a propane fueled steam powered cycle.

The pressure of saturated steam is directly related to steam temperature. 120 psi is 350 deg. F. I would think you could adapt a gas oven thermostat to regulate the steam at 350 deg.

piston ==> valve ==> spring.
Supply the piston with pressure from the boiler, and arrange it to close the valve. Crank the spring down for more pressure, let the spring off for less. (Come to think of it, you should be able to use a diaphragm instead of a piston, like compressed-air regulators do. The difference is that instead of the diaphragm actuating a valve that supplies itself with pressure, it'll be actuating the gas valve).
Expect imperfect operation in the form of lag and pressure variations: if the thing's just sitting there steaming gently and you nail the throttle the boiler pressure may go down quite a bit before the burner makes it catch up. Similarly, if you ask for extended high power output (climbing a hill, or going fast), then the boiler pressure will sag and stay sagged until you park the thing again.
You can fix the long-term sag somewhat by making the valve more sensitive, until you make it so sensitive that the lag induces oscillation.
But as long as you've got wiggle room above and below 125psi, you should be OK.

--Many moons ago Coles Power Models sold a widget that did just this; not sure if they're still around or not but you might have a look. Maybe ebay too.