I cut a chain as short as possible to fit the gearing I expect to use at a given track, plus or minus one tooth.
I have several chains and store them with the sprockets.
At thirty bucks a pop it’s a worthwhile investment.
Also, check to make sure the motor mount isn’t sitting on top of a frame weld and that the airbox is clear. You need to be able to move the motor & mount freely back and forth without feeling any “bumps”.
There was one guy I saw lose his chain every session.
When that happens, something is wrong and one must find the root cause.