First off, this has been discussed in droves, try a search and see what others have said about this subject.

Otherwise, it is up to you and how you are going to use your GT. Are you going to race it? Bash it? Or both? Are you on a budget or is money no object?

If you plan to race at your local track, .12 sized motor is probably the only way to go, legal at the tracks.

If you plan to bash around at the park, neighborhood, quarry...whatever. You can go up to an .18 sized motor.

Not too many motors made with a short shaft, but, you can cut the shaft of any standard shaft motor.

O.S. makes some great motors for the GT. O.S. is the standard for reliability and ease of tuning and boast some great high output motors.

http://www.osengines.com/engines/car.html

The following 3 motors are drop ins.

most legal horsepower from OS... [link=http://www.osengines.com/engines/osmg1995.html]12TZ(s)[/link]

Great reliable legal motor... [link=http://www.osengines.com/engines/osmg2045.html]12CVRX(s)[/link]

Budget torque monster...[link=http://www.osengines.com/engines/osmg2105.html]18CVRX(s)[/link]
regear this thing and don't look back, not race legal. What I got.

Budget minded...not worried about horsepower...About the same as the stock AE .15... Need to cut output shaft...[link=http://www.osengines.com/engines/osmg2010.html]12CV or 15CV[/link]

Also, take in account for your exhaust. The TZ is a rear exhaust motor and will need a new header to replace your stock header. The other three are all side exhaust and can use the original GT header.