A lawn mower engine would be excellent for powering a mobile band
organ, as long as you choose the right one.  You would want something
with built-in gear reduction if driving a fairly small pulley, and
a horizontal output shaft, of course.  Locke mowers used this type of
engine for many years, and replacements are available from several
manufacturers in different sizes and horsepower ratings.  Piping the
exhaust to a muffler under the trailer should solve the noise problem.

I think the ideal gear ratio would be around 6 to 1, with the size of
the small pulley (which would probably have to be custom-made for flat
belting) determining the speed at which the organ would be driven at
a reasonable engine crankshaft RPM.  One source of such a new engine
would be J&M Mower in Rhode Island:  http://www.jmmower.com/

I myself have such an engine, hardly used at all in spite of being
about 30 years old.  It's available for practically nothing considering
that packing and shipping could cost about what it's worth.  Having
sat so long, you would have to clean the carburetor and replace the oil
with perhaps SAE 20, run it about an hour or so and then change the oil
again, this time using the recommended weight, probably SAE 30.  I would
prefer to sell to someone who could come by and pick it up, or send
someone else after it.

Jeff Wood
Wilbraham, Massachusetts