Jim,

All Cox .049 engines have the same stroke, so the TD cylinder and piston should not require much in the way of head shims over a stock TD.

Here is a great article by Paul Gibeault that may help you to set up your GB properly. You will probably have to drill out the venturi to get the best performance.

http://coxengines.ca/files/MRP.pdf

Good luck with your hop up!

Bill