The SG 6042 has 10% thickness and 3.8% camber. The Clark Y has 11.7 % thickness and 3.4% camber. A quick look with Profili says the 6042 produces a max Cl of 1.5 at Reynolds number of 100k. The Y max Cl is about 1.3. So if you can slow the plane down to 20 mph with the 6042, you would "only" be able to slow to about 21 mph with the Y. The slowest speed the plane can fly goes as the square root of lift coefficient, Cl. So a 10% higher Cl with the 6042 gets you a little slower minimum flying speed but not much.
I would make sure the 6042 does not have a nasty slall tendency. The Y is used in lots of models and is reported to be well behaved.