There is a lot of activity these days in solar power systems for off-grid locations, using reciprocating steam engines to generate power and hot water storage systems to provide 24/7 service. A company named Terrajoule is claiming to have a modular system providing 2 mW generating capacity.

The utility of such a system however may extend to individual residences where utility power is not available.

Are there any efficient, small scale uniflow steam engines available in a capacity of 5 kW?

Terrajoule is long gone, you can see their engine at Tom Kimmel's shop during the SACA fall meet.

At the moment, I have no idea where you would find an efficient uniflow engine "off the shelf". Tom also purchased the patterns to make Gary Hadden's two cylinder, double acting uniflow but there's no machined engines sitting around ready for sale … at the moment.