If you are looking for a cheap, tiny engine that has reasonable horsepower per litre, you might try looking at the ones used on Honda postie-bikes over the years. 50-110 cc, depending on when they were made, they are OHV with centrifugal ignition advance, and put the mower engines completely in the shade both for output and durability. They are also dirt cheap, so long as you don't mind taking a whole bike and tossing away what you don't want. At farm clearing sales they seem to be in the $50 range.
