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Power plants typically use the Rankine cycle with steam turbines, they can get a bit complex with super heating and reheating stages but its all based on the simple Rankine cycle. Smaller scale power plant with more intermittent use might use gas turbines or even diesel engines.
I'm sure you can build a working turbine at home, but it will not have the same eff as an off the shelf PD motor that is converted for steam. If the air drill took 4cfm at 120psi to turn the alternator would a DIY turbine take less CFM to do the same job? Not likely. A turbine is a vary simple machine but to make it eff one must pay close attention to the shape of the the turbine blades. Like how tweaking the aerodynamics of your car can greatly affect FE the same is true for the turbine blades but to a greater degree because of the higher velocities and the importance of the force vectors.
Someone mentioned earlier about using a old lawn mower engine by simply welding a second set of lobes on the cam. This is a good Idea and probably the easiest way to make a high eff motor. Ideally you want to let in the high press steam close to TDC then close the intake valve and let it expand to BDC. So in addition to adding extra lobes It would be ideal for good eff too grind the intake lobes to only open for a short time at TDC. The steam motor will have a expansion ratio that is the same as the compression ratio, so on a typical engine you can expand the steam 8:1. This would mean that on a open system 118psi would be the optimal press anything above that press will have leftover steam press at the bottom of the stroke anything under and it will over expand creating a vacuum. At 118psi and 8:1 the pressure approaches zero as the piston approaches BDT, this might be optimal eff but there must be a point of diminishing returns so having the pressure a little higher wouldn't be a problem. Also if you want to run higher press more eff you just have to increase the compression/expansion ratio.
The two stroke motor wouldn't be vary eff because full pressure steam pushes the piston unit the ports are uncovered and the steam is released at full pressure. The motor has to compress the remaining steam as it returns to TDC. Plus how do you lube it.
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