EDF's don't generate thrust quite the same as props so it's hard to pin down an answer. EDF powered models that can barely take off from the ground because the fan blades are starved and operating in a near stalled condition end up flying far faster than equivalent prop powered planes in flight once the fans are being fed with adequite airflow and the blades are not operating in a stalled condition. Prop planes with high pitched "racing" props have some of the same issues. So there just isn't a way to pin down what you're looking for that I know of. That leaves you with just basing the performance on published real tests of motor and fan unit combos.

You're doing the right thing by copying an existing design. But you're going to get into trouble by making the wing area smaller. Instead scale up the span and length enough to keep the wing area to within 10% of the other model. But you'll also need to watch the weight. You must keep the weight down to no more than what the other model weighs if you expect to match the performance of that model. Or if you know you can make your model lighter enough that the wing loading on your smaller wing is the same as the other model then by all means carry on. By making the smaller wing have the same wing area it'll tend to crank around in the turns just as well and the lighter weight will assist with climbing and recovering speed after high G turns where a lot of speed is bled off.

Other than this your best bet would be something like Motocalc or similar that has some fan and motor options to select from.