Nimffish,
Here's something to consider. Even though a 50cc engine is lighter than a G62, if you get an engine like a DA50R, the motor will actually sit further in front of the firewall due to the rear mounted carburator. This may offset the difference in the weight of the motors.
The more I think about this post as I type it, the more I realize that you already know this.

Good luck with your plane and keep us posted.
Happy New Year