There have been a number of similar engines done. Of course with that tank you would only get about 30 seconds of run time.

I could whip one of these up in about 15 minutes with photoshop...   

When I was in High School the father of a good friend built two versions of this.  He built a two .049 (.098) and he built a twin .15(.30).  The twin .o49 was installed in a L-19 Bird dog early R/C plane.  The radio failed and the plane flew away never to be seen again.  I think because of that event he never put the twin .15 in a plane.  He actually worked with Cox during the development of both engines.  Here is a bit of trivia the bottom end of the connecting rod was partially removed and the rod was pushed over the crankshaft throw.  His idea, and it did work, was the connecting rod always pushed against the crank.  Since the rod never pulled on the crank it just required enough pressure to keep it attached to the crank.  Weird idea but it did work. 

One last the thing, the guy who made these engines worked at the time for a company named Drackett.  They made Windex window cleaner, the guy claimed to have invented the push spray pump to spray the window cleaner on windows.  At least that's what he told me.
Andy