The 9x8 on the Rossi sounds good, may try a 9x9 also
I had the Lanier F-4 with a YS-45 and it flew alright.... for looks it was great but for speed it was so-so. I am not familiar with the Enya-50CX, but the Rossi-53 is a monster! If you go with the Rossi I would use the short pipe because it will not mess up the scale lines of the F-4, the long-pipe is obviously gonna have better performance-
Do you already have both of these or are you engine shopping?