Re: Nell Lusso
Welcome to the forum, Gus. There're many who have built successful bikes from the big box stores. Target, K Mart and Wally's place offer some good deals on bikes. Yes, there are those who shun the less expensive Schwinn, Huffy and others and they have sound reasons but don't let that spoil your fun.
As for using rubber in the engine mounts; much has been written here regarding that theory. The general consensus, and one which I personally subscribe to is that you can not eliminate engine vibration but in fact simply transfer it to the engine mounts and fasteners by mounting the engine in a resilient material. It might 'feel' better to your body but somewhere down the road those parts mentioned will suffer. Mount the engine solid and live with the vibrations that are inherent in a 2 cycle, single cylinder engine.
As for the chain tensioner/idler, you don't need to go to a shift kit. Use the search feature at the top of the page, key word, tensioner, and you'll see lots of suggestions that might help.
Good luck, have fun and ride safe.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"