I think you'll be pushing it. A Ryobi 31cc would be good for a true 1/4 scale Cub, but even then you'll need to plan extensively on how to mount the engine. with a motor mount, the Ryobi is more than 7" long from back of mount to prop hub. Another thing to consider before you modify the kit to fit the engine is prop clearance. The Ryobi likes an 18x8. You can go down to 16x10 but that's about the smallest prop you'd want on the Ryobi. Make sure the GP Cub kit can clear a 16" prop with ease.

If you've got the kit NIB and the Ryobi already, give it a try. Otherwise, I would recommend against it. I've got a couple of WM 1/4 Cubs - one full wing and one clipped. My Ryobi with CH Ignition will fit within the cowl, but I'll have to mount the engine such that the carb sticks through the firewall. This means that I'll have to cut a hatch cover to get access to the carb adj and throttle linkage - not to mention another firewall behind the carb to keep the gas sprays and spillage off the radio compartment. I am going to use one of my smaller Echo 25.4cc engines instead. Sure the carb and muffler might poke through the cowl a little, but it'll be a direct bolt on to the existing firewall.

For your reference, I just finished up a Sportsman Aviation Super Decathlon (86" span 1.245 sq.in. wing area) with a Homelite 30cc. I've also got CH Ignition on this one and it balances out the 15lbs plane with no lead needed. The weather here lately has been crappy, so I haven't had a chance to test fly it yet. I expect it to have just the right amount of power for scale aerobatics.