I'd like to throw my two cents in. I have used the stock chain tensioner on all my builds. I have not had any problems with them that were caused by their design or manufacturing. I was always the problem.
I can give this advice: listen to these guys in this forum, hear what they have to offer, keep an open and creative mind. Also, start from the very beginning of your build with accuracy and precision.. as best as possible. Mount your engine correctly and in the proper position. Mount your sprocket to a true and symmetrical tire/wheel. Align your engine sprocket and wheel mounted sprocket precisely. If it's not, disassemble and start over. Continue to start over until your sprocket alignment and sprocket trueness are perfect or at the very least within the manufacturers specs. Then cut your chain to fit. Use your tensioner to apply proper tension to your chain and constantly observe this area of your bike for changes or misalignment.
I have found that it's usually not just a chain problem. It's a problem with everything you did wrong up until you put on the chain. Go back to the beginning and do it again. I am being insistent about this because I have spent uncountable hours pulling my hair out trying to fix my chain alignment when it was the sprockets or motor placement. Could save a lot of time by doing it absolutely correct first.
And always always listen to and respect these well-known members. They know their stuff and are willing to help with your motorized bike.