I will be unveiling a new boiler I finished only a few hrs ago at tomorrows Dorkbot.
laughingsquid.com/dorkbot-sf-44-at-the-gallery-with-no-name/

The boiler weighs about 25 lbs, and at this point puts out about 1/2 hp
After doing the math it would appear it should be able to put out about 3-5 hp with the right burner, With some work I could get it into a large bike frame.
The nice part is it takes only about 3 1/2 min to go from dead cold to 45psi.
I will be demonstrating it at the talk.

Kimric