Strictly speaking you don't really need a pattern plane to practice F3A Pattern. You just need to make sure the design you pick is aerodynamically efficient enough to fly well at higher speeds. This usually means a slender fuselage and tapered wings. You also want to avoid the hardcore 3D type scale aerobats with HUGE control surfaces. Flying those at pattern speeds is inviting disaster. Pattern planes are purposely-built with thinner airfoil for carrying speed through manuevres and long tail moment for stability and controllability without the drag penalty of large tail feathers. Pattern flyers usually use higher pitch props for that reason.

It sounds to me like you are not really looking for an F3A "Pattern" plane. You just want to use your Ryobi 31cc on an aerobatic aircraft. If you stay magneto, you are limited to something like the Stinger or 4*120, as the weedie won't power the typical Extra, Edge or Giles with any kind of authority. If your 31cc is a twin-ring and you upgrade to elec. ign and a big bore carb, you might be able to shoehorn it into a 1/4 scale scale aerobat and do well.