Look at the Zeonoah engines and their clones.

They run on regular petrol/gasoline (95 RON unleaded recomended) mixed with oil at a mixture of 25 parts petro to 1 part oil.

If you want to have it running for a very long time with minimal load, then look at the lover powered variants, like the G23LH and GZ25N
http://www.davesmotors.com/store/product346.html
http://www.davesmotors.com/store/product1031.html

If memory serves me right, I have seen Light Weight Generators powered by both of these.
They are blower cooled and should have a run time of several hundred hours, before needing service.