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

I would definitely go with a proven design for your engine.
There are plenty available as casting kits and from drawings you build from bar and round stock.
They are a lot work either way and when well finished, your reward is a great looking fully functional piece of hardware.

Regarding steam expansion, more heat is your friend.
Cooling a charged steam cylinder will be the death of your power. The faster it cools, the quicker the stored energy is vanquished.

Temperature drop is not what makes the pressure drop. It is the pressure drop that makes the temperature drop.
Steam can exist at 212 degrees F but only at atmospheric pressure. For example, steam at 120 psi is 354 F.
You can find charts showing steam pressures with the corresponding temperature.

We are looking forward to following your build so please keep us updated.
Best of luck with your ambitious project.

Gil