Well, anything could be a 250W motor. What's the diameter of the hub. around 100mm? What are you getting for top speed out of it? What's the continuous current out of your controller?
A Bafang BPM or Bafang CST (if you want cassette) might be a nice upgrade. Hub diameter around 160mm. I don't own one, but I do have a 500W Bafang fatbike motor and that easily gets up to 25 mph on 48V. I probably don't know what I'm talking about, but you probably want the 201 rpm version for 48V.