Plug the holes in the fuselage/cowl mount blocks with toothpicks & CA. Make a shim of two thicknesses of cereal box cardboard or put two popsicle sticks behind the spinner (one on either side of the prop shaft) and re-drill 1/16" holes in the firewall blocks for the cowl attachments.

I don't specifically know how the Sundowner cowl is attached, but you get the drill.

I would NOT change the engine to suit the cowl. Ever.