In the assembly manual it states " If you are using a 2-stroke engine such as the Irvine 1.20 or 1.50 or a 4-stroke such as the Saito 1.20, 1.50 or 1.80 we suggest using an opening which measures 6.5" long from front to back, 6.5" wide at the back edge by 3.5 " wide at the front. This allows room for the pitts style muffler and adequate cooling area, with plenty of air for 4-stroke engines." From personal experience I can tell you that failure to give a large displacement engine adequate cooling area is going to cause you big problems. I know it don't look so good but that engine will seize without enough room for the air to exit the cowl.