It is always best to provide a link to the plane you are asking about.   Is this your plane?
http://www.electrifly.com/largeelectrics/gpma0110.html 


In any case, there is a good book at the top of this forum that is all about electric flight.  You may wish to take a look.

But, assuming your plane is 3 pounds with the stock motor and the recommended battery, then we will use that as our weight target.  A new outrunner brushelss motor and Lipo battery may be able to shave a few ounces off that number.

Your target power to weight ratio should probably be between 60 and 100 watts/pound.  I will use 75 as the target since this is a trainer.  So you are looking for something in the 220 to 250 watt range.  Assuming a 3 cell lipo, at 11 volts, that would suggest a motor/prop combination that will draw in the 20 to 23 amp range. 

A 1000 kV brushless outrunner with a 10X7 prop would be a good starting point.  Or a 1200 kV with a 9X6 would work too.

A 2400 to 3600 mah 15C or higher rated 3 cell lipo pack would probably be a good target.

Naturally you can go more powerful, but I would not recommend going less.