I have given serious thought about replacing the ST .46 with a ST .51.
Mine started life with a very good ST 46 but just wasn't quite enough. The retrofit was a bit of a challenge but it came out pretty well. I drilled another set of rear holes in the 51 to match the 46 mounting holes. The engine shaft is a bit longer, had to go to a Dubro plastic spinner with the recessed back plate to make it look right.
The skylark is a great stunt ship but has a thick wing and needs a really strong engine to do it justice. If your Skylark is the same wing area as mine your wing loading is 13.37 oz/sq ft. That isn't too bad, I know one expert from Dallas that says he builds for 13 to 14 oz/sq ft and feels it helps in the wind.
You are going to add two ounces with a 51 which will push the wing loading to 13.84 oz/sq ft. still not bad enough to make it a wall hanger and with the extra power it will be fine. You will need at least a 5 1/2 ounce tank and maybe a little tail weight depending on how it balances with the 46.
Good luck and hope to see it at a contest some day..
BTW: The best flying stunt ship I have is 13.83 oz/sq ft.