I don't think you will have any problems with your Super Tigre GS 40, bearing in mind this plane was designed for a 40 sized engine and todays tendancy to overpower airframes. - We have a guy at our club who has a 75 engine in a cub designed for 40Four stroke, he can take off in 4 ft and knife edge - in a CUB.
I have a Saito 56 (four stroke) in mine with an APC 12x6 prop, rarely <u>need</u> to use full throttle but I do anyway
Anyway with the open nose of the four star it would be really easy to swap out the engine for a 46 or 55AX when you aquire one - should you feel the need.