Sometimes this will happen depending on how the crossbar behind the seat is positioned. One of the differences with “four stroke chassis” is the position of that bar.
It’s not something to worry about other than being cosmetic, assuming you don’t run it for so long you saw through the rail 
Some remedies:
Move the clutch drive outboard (assuming it might be inboard already)
Wrap something around the rail where the chain is hitting. A piece of heater hose, cut, placed over and zip tied is surprisingly durable. Solid nylon or similar even better.
You can play with sprocket sizes too. Run larger driver sprocket and smaller axle for a given ratio.
Re-locate the rear crossbar. Kinda extreme, but people would often do this with CIK chassis 5 or more years ago before 206 marketed chassis became popular.