You can just do things. In this spirit, I am attempting to chronicle building a steam powered Ethereum node, API and website.

Speed: 5300 RPM
Voltage: 10 VDC @ 10 Watts
Voltage no load: 12 VDC
Max Current: 2.4 Amps

So I got 10 watts to work with and that's maxed out. So not much. 
Here's a pic:

To get the 5300 rpms out of the generator, 

½” pulley on the dynamo.  

For our engine, the flywheel is 2.5".  5300/5 = 1060 rpms roughly.

These steam engines typically run at 500 RPMs and the boiler works hard for it. 

So finding an engine with 6.5" dia. flywheel would be ideal. 5300/13 = 410 rpm roughly