You need it to produce 7-8hp around the clock for 17.5k hours? If that is the case, the closest that you'll be able to get is to use an industrial engine that is built to handle 20+ hp at a high duty cycle.
The Kohler Command Pro (CH740NG) is a very over-built industrial engine with a...
This brings up an interesting idea: Optimizing a mixture of two different types of fuel for different engine speed/load combinations to optimize combustion flamefront speed and/or reduce knocking.