120 2 stroke with a pitts muffler would fit fine, and would fly the dog out of the Z. You would have to put all weight possible up foreward and maybe some lead for balance. Flew my Gee Bee Z a week ago and on landing, at the end of the roll out, the throttle suddenly went WFO. The Z started to take off again by itself. I leveled the wings and pulled back gently and the Z climbed back up into the sky, the throttle linkage had come apart! My friend Mel Santmeyers and I took turns flying the Z until it ran out of gas, about 40 mins later, and I dead sticked the Z back down on the runway safely. The nylon piece that connects to the Sullivan ball link was gone.... The Z flew great thru it all.