The only power meter that will bolt onto an FSA MegaExo crankset is a set of power meter pedals (as pedals are compatible with any crankset). There are no other power meters that will fit a MegaExo crankset. Stages Cycling used to make a MegaExo crank arm but discontinued it around 2017.
Please also note, if you decide on a new crankset-based power meter, we recommend changing your bottom bracket. MegaExo cranks use a spindle size that is 24.05 mm in diameter, vs. the more common 24 mm and 30 mm spindles. So your new power meter crankset will come with a 24 mm or 30 mm spindle and will need a bottom bracket to match.