A picture would help identify you mystery device. Any manual decompression valve I have seen is operated by a small lever, on the handebars. You pull in the lever, give it a good kick and then release the lever. I have never seen a decomp on a motor that small before though. Always a good idea to take a look inside the carb, if it had been sitting for a long while it will need a good cleaning. Good old ebay is my favorite place to buy parts, just make sure to only buy from users with a good seller rating. You can find manuals there too.
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Yours truly, sincerelly, love always, Josh aka The DirtDemon.