I have both the Tiger 2 and the Tiger 60...They are fantastic flyers...Mine are not the ARFs , mine are the kit models...The 46 2 stroke motor is a very good match for the Tiger 2...I like 2 strokes in tigers...The 91 is a little big for the Tiger 2 although they do usually need nose weight...The 91 is going to have a lot of torque and you may find the little Tiger dosent handle it too well . The 91 would suit the Tiger 60 much better although I would use a OS75 two stroke.....I think you might spoil a real nice plane by putting the 91 in but yes I think reinforceing the firewall would be a good Idea...I am not sure how tough the ARFs are but I know when I build a kit they come out pretty strong...I would not be afraid of putting a 91 in one of my air frames but sometimes ARFs glue joints are not the best...I honestly think you would be better off to leave the 46 in your Tiger and enjoy it . If you want a plane for that 91 either build one or buy another ARF....I would be thinking about a 60 size Cub would be a real nice match ...A Tiger 60 would fit.......