I'd go to the LHS and buy a piece of balsa that most nearly meets the same density, width, and thickness as the one that has the curve in it.
Cost maybe a couple of dollars. make a copy of the spar that is straight and cut it out replacing the bowed one.
I never had luck trying to straighten out a curved piece of wood. I've been building since the mid 1970'sYou'll spend more money on buying the stuff to correct it and waste more time than cutting a new piece..It may not even straighten out all the way once the pins are removed......Ron