Is there a regulation that addresses if a unit loses a DA Form 705?

A COA that I would recommend, as I am not aware of any particular reg about lost 705s, is the Training NCO go into DTMS, get all the scores, and create a new 705 with comments/annotations like "Reprint due to lost original" and have someone present at the time of creation of the copy. Perhaps the 1SG.

There is most likely no regulation to solve the proplem. Maybe the chain of commander can recontruct it or have the Soldier retake the PT test and make sure there is a copy. Regulations do not cover everything. Common sense has to cover thoses that don't.