RE: Quique Somenzini YAK-54S 73"
Feel free to cut the center hole out as much as necessary to provide good clearance, and then of course you will open the cowl area that is directly in front of the head. You might very well have a "keyhole" shaped hole where the circle and head opening ome together. If possible it's nice to have them seperated by some material to help keep things "tight" but certainly is not necessary... Most Yak cowls are completely open...
How far out from the cowl (not the hole but even with the very end of the cowl on the outside) will it protrude? Full scale Yak's have them out there a good bit...