In response to Locktite,  both of those engines will run very well if you get a good one, but doing so is a bit hit and miss.  The slightest leak in the crankcase, and you're doomed!  Hold them to your ear and turn them over slowly, and if you don't hear a bit of a "pop" as the transfer opens, there's a problem.  In fact, a number of the Clan 0.24 cc engines had a manufacturing error - the transfer system was incorrectly machined in such a way that the transfer port would not open at all!  If you have one of those, you're out of luck! 

Another essential is a good compression seal.  As Motorboy says, getting enough heat into the cylinder to achieve a start becomes increasingly  diffiicult with decreasing size. The tiny engines can tolerate far less wear than larger models - a very small amount of piston wear or surface scoring can lose you all of your compression. Keep 'em clean!! I have a Bambi which I've now had to rebore twice.  It still runs fine, but it does show how little wear it takes to render these little engines unuseable.

Another thing that can prevent a tiny engine from running is the presence of castor gum in the induction or bypass systems.  This is not an  uncommon thing with engines that have been run but were not properly cleaned up after being run. Larger engines can usually tolerate a bit of this, but not the really tiny ones. The only fix with a gummed-up miniature is complete dismantling and a thorough clean-up.

My examples of  both engines run beautifully.  It's a bit of a lottery ...................