on the slow rpm engine you will probably need 2 or 3 degrees of right thrust,to put offset in before or after you fly the plane is up to you,unless the planes call for offset I'll fly the plane then trim as needed.With G23 you'll get about 3 times more flying time on the same amount of fuel as with the 120 engine,fuel tanks are interchangeable, the fuel line,tank stopper,tank vent lines and clunk lines need to be changed to gas. I try to set my fuel tanks as close to the c/g as practical,I would leave the c/g where it's at,then experiment after the plane is flying again