Dremel out front of chin cowl for engine cooling, then dremel out clearance for YS connecting rods/pump. Trial fit chin cowl. Guess what, yes, it fits just about perfect...

Trim balsa from around back rear lower deck. Sand flush with a block to make sure its straight and to keep from sanding into the foam mating surface.

Mark the spot where you cut the taper in the front to conform to the rear chin cowl.