To be honest the firewall souldn't need reinforcing with a 26cc gas engine. They aren't that different to the 120-150 size four stroke engines the plane was built for. The re-inforcing was done with the 55cc due to large amounts of thrust/torque. If you do strengthen the firewall at all, use heavy cloth (around 6 oz) and resin. You want to build a fillet type join between the back of the firewall and the sides of the fuselage. The ply firewall is simpy glued to the back of the fibreglass firewall at the factory. By strenghtening the join around the edge you are trying to spraed the load from the engine into the fuselage. Without strengthening and a large powerful engine the firewall will eventually stress crack and break away for the fuselage. Cheers,
Dave