I did come across a very small water-cooled engine:
And here are the specs:
46.5cc,2-stroke,water-cooled,pull-start gasoline Engine
Specifications:
1) Engine type: single-cylinder, 2-stroke, water-cooled
2) Piston displacement: 46.5cc
3) Bore x stroke: 39.5 x 37 mm
4) Max. torque: 5.8N.m/12,000rpm
5) Max. power: 6.0KW/11,000rpm
6) Min. idle rotate speed: 3,500rpm
7) Compression ratio: 9.0:1
8) Ignition: CDI
9) Spark plug: Torch
10) Carburetor: diaphragm
11) Air filter: polyurethane foam
12) Start mode: pull-start
13) Clutch style: adjustable centrifugal automatic
14) Average fuel consumption: 610g/kWh
15) Dimensions: 400x210x230 mm
16) N.W.: 5.5kg
So if the max power is 6 KW, then it might be reasonable to expect about half of that (3 KW) on a long term basis.
If the unit was mated to a pancake generator similiar to one that the Honda CNG uses, the generator would also function as a starter motor. My hybrid car has a similar generator built into the flywheel, that functions like a starter, and it works better than any starter I have experienced.
So if the electrical generation was around 15% efficient (not unreasonable), we could get about 450 Watts of power. And if we could capture 60% of the 'wasted' heat energy, we would have 1530 watts of heat energy or 5220.36 BTUs.
Those energy levels would be in excess of my non-peak home energy consumption, which means I could sell energy back to the power company.
Not bad for a 46cc engine.
-AC_Hacker