Hi, so many variables here and very difficult to advise but I did a one off experiment using a Buhler motor with my steam engine,. This motor produced quite a good output. I did not clock the rpm but the maximum I could screw out of it was 9Volt dc @t 1/2 amp... thats just over half a watt. This was on my Vtwin oscillating engine, double acting, 15mm bore, 20mm stroke, running flat out at something like 20psi. For fun, I hooked it up to my transistor radio, using an electrolytic capacitor across the output and tweaked the speed of the engine for 9volt at the few milliamps drawn by the radio and carried on working in the workshop with this lot steaming and whizzing away in the background. After a couple of hours of this, I realised the thing had consumed huge quantities of gas (poor boiler arrangements) and was costing me a fortune but it proved that these motors were perfectly capable of producing a useable output. So, in answer to your question, yes it can be done, it is very wasteful but who cares, it's fun to watch.
Perhaps some of the other chaps have more detailed figures or may suggest a better motor/ generator than the one I happened to have sitting around.
Details of the Buhler motor come off the label on the casing and are as follows: 127k26960 ... 1.13 .043. 263 .00
Motor length... 3" (75mm)
Motor dia........1 5/8" (40mm approx)
Experimentation is very much the name of the game here... great fun even if the outputs and efficiency figures are totally lousy.
Good Luck....Chris
Supplied by Model Motors Direct (don't know if he is still around)