i believe it's the same plane as the skyshark fantasy. use to quite promoted on RCU but has been quiet for a while. it is an older design that skyshark now kits for the diyer.

http://www.skysharkrc.com/fantasy.htm

you will see that your engine is not even in the recommended range. although i am not an advocate for overpowering planes, i do tend to lean toward the larger of the recommended engines. i would say at least go for an OS FS-70SII or a saito 82 both 4-strokes. i think you'll have a really hard time shoe-horning a 91 in there but you can try. a magnum or tt 91, both great engines. for 2-strokes, a 52 would be fine but i'd go for the O.S. 46AX... one of the best engines available. a ~75 would be good, too.

but it would also depend on how heavy your plane is. each builder will tend to build differently. my attempts at building were always a lb over the designed weight. made for less than stellar performance :-)

what intrigues me is how you have the plane balanced with a 40? would seem to me that it would be very tail heavy...