I made one up easily: I used the Baja brand roller and brass colored arm from Tractor Supply co, along with the factory spring that goes with it. I used a long machine screw to replace the stock clutch cover screw, which acts as a pivot point for the roller. I used a piece of 1/4 clear plastic hose ( a piece of bic pen will also work) and washers to "re size" the screw so the tensioner wouldn't rattle. I used 2 heavy duty zip ties around the seat tube to pull the spring to tension, so the rubber wheel pulls down on the top of the chain.
If your Iranian friend does not specialise in motor bicycles, perhaps you should give another shop an opportunity to work on the bike, or see if you can copy an existing design from this forum.
I like the long bracket and plastic roller idea (use a piece of flat bar to bridge the seat and chain stay, with a slot to mount the roller; it works like the stock clamp on tensioner, but can't turn into the spokes)
If I ever build another of these grubee type bikes, I'll just weld 2 stock tensioners on the frame (one on each side, unless it has a derailleur).
Good luck.