Re: replacement parts
There is a post on this site where a guy made his own spring-loaded tensioner out of a coaster brake arm. Also, bike shops (and EBay sellers) sell spring loaded chain tensioners for single speed bicycles that clamp around the chain stay of your frame. That may cut down on how often you have to adjust the tensioner that came with your kit.
Check your chain carefully to see if there are any bulging, stuck or bent links. That could cause the chain to wobble off of perfectly good sprockets. It's easily fixed with a chain tool or a bicycle shop can do it. Good luck.
“You're only young once, but you can be immature as long as you like" -Unknown