Be aware that with a mid-drive you could have a major problem with your chainline, since some/most/all have a much greater crank to chainstay distance on the right side than the left. Those are steel forks and if the distance between the dropouts is 100 mm, I see no problem with a 250w system as long as you use one, preferably two torque arms. Read about this in the tutorials at ebikesca. There are small motors which fit narrower dropouts, but you'll need to research if that's the case.