Gary, Where I work my boss got hold of an 80hp jet engine complete with gear box to a 5,000rpm output shaft. A little over the top but weighting only 20kg could be possible (execpt fuel capacity). I belive it was a 747 starter motor . Even if they let you use enough fuel the exhaust would be so hot it could be classed as a weapon and disallowed and its not classed as an IC engine which is what it states in the rules as being allowed. Its impractical but then again....

Another posssiblity that crossed my mind was a steam powered robot, know a few people who build them. Not been done yet to my knowlegde.