Yak-9 and Yak-3 are much smaller than Mustangs and Spitfires. Originally, they used Klimov M-105 engines (Hispano-Suiza 12Y derivative). Currently flying examples use Allison V-1710 because they are close enough in size, weight and power - and are available in usable condition. I've never heard of a Merlin installed in one; I think a Merlin would be too big, except possibly for building a Reno Racer.