GET A FREEWHEEL WHEEL-
I NEVER use a tensioner- one build I have the chains evened up pretty well- but it takes some messing with- another thing is that CHAINS STRETCH-
even playing around with half links, they get out of sync, because the motor chain will usually stretch more than a new pedal chain- If I use a half link, it is easier to split and size the smaller pedal chain-
But if you put a freewheel on the pedal side you can hang a derailleur and the derailleur takes up the slack on the pedal side so you can first just set the motor's chain-eliminating the heavy unreliable tensioner and if there are no clearance problems otherwise
You can also have gears then to pedal if you want- I've just put on a three cog freewheel they sell on ebay- search "freewheel" and list low to high- it is less than $11 delivered- and small and light 14, 16 and 21 teeth- or you can get a 5, 6, or 7
I wanted a lightweight one- i got this a while ago and am putting a 27 inch wheel on the back for easy peddalling in FLAT Florida.
Other "tensioner Eliminating tricks"- use washers and spacers on a freewheel hub to move the motor sprocket side to side- the derailleur adjusts
Get a smaller sprocket- the smaller the sprock- the more chainstay clearance- I'm just putting a 34 on but the 36 worked much better than a 41 and much much better than a 44 especially for clearance
A cruiser frame has the most chain stay clearance, then a mountain bike and least of all a road bike- generally at least there may be exceptions.