chain tensioners

Hi folks, New to here so thought I would say hello. I got myself a pretty awful $90 chinese morotized bike from banggood and I'm trying to fit it to a 18in Kona Hanana frame.I'd appreciate some advice - although I've managed to overcome some problems, I'm struggling with the chain rubbing on...

has anyone ever used two sprung chain tensioners (real ones with teeth), one on the top of the chain and one on the bottom, to eliminate the poor design of the original ones that come with the kits. Having one on the top would keep the tension on the chain when you aren't giving it gas.