Hi HSteve, and welcome to WattFlyer!
Let me start with a disclaimer: I'm certainly no expert in these matters. I did, with lots of help from here, and with a mistake or 2 along the way, upgrade the power system of one of my planes. (Well, 2. But the second one I just got a package motor/edf. Doesn't really count.)
I'd start with getting the weight of your plane. The weight of the new power system, motor/esc/battery, will be different, but this is a good starting point.
Then start reading:
Here's a thread just about powering your model, including the list of desired watts/pound:
And then ask alot of questions! I highly recommend shopping at places that give specs of what power a given motor will give based on battery and propeller size. HeadsupRC is one that was recommended to me, and I like it.