Right spoke to my dad who owns steam engines...
If your using a ossolating engine then you cannot control the steam stream with a regulator as it applys to much back pressure to the boiler ( basically it will blow your line or the boilers safety valve)
The only way to control speed using a matched boiler and ossolating engine is with a variable pitched prop....
If however you are using a slide valve engine then You can regulate the steam input safely as the engines are designed that way specifically with the valves already built in..
The prop shop used to do variable pitches props but no longer in business you would have to look on eBay or import state side.... be prepared for a shocking price, variable pitched props are very expensive bits of kit....
My dad also says you can reduced speed ( but it’s fixed becuase you can’t change it during running) by running as a lower psi but you may not get the torque by doing so.
All Info comes from my old man who’s grown up, built and owns steam engines with 60+ years of experience.
Hope it helps
I'm Not Anti-Social, I'm Anti-Stupid.