1/4" is overkill for something like this. Four #10 bolts, hardened or not, are plenty strong enough for even the most powerful Ryobi 31cc.

How strong you bolt your engine to the firewall should be matched by how strong your firewall is tied to the rest of your airframe. Most of the crashes I see, the firewall always get ripped out of the fueselage along with the engine block.