Unless the existing firewall is damaged in some way, I wouldn't worry about adding the plywood in addition to the firewall, you can just redrill to match the mounting pattern of the mount turned 90 degrees. The stock firewall is plenty strong for that motor. If you need the engine to be further forward to clear the cowling, then all you have to do is mount the engine further forward on the mount, or use a slightly larger mount.

I agree with Tbird on not using wood screws, use bolts(with lock washers) and blind nuts, and/or self locking nuts if necessary due to engine vibrations.