I'm building a busa 1/4 scale J-3 right now. The front end is beefy enough that you should be able to move the firewall back with no problems. Theres enough room inside (right behind the firewall) that you should easily be able to get your hands in to mount standoffs from the firewall to a backplate on the engine (with the fuel tank removed). I was thinking of doing this in a future mount and running some 8-32 or 10-32 all-thread from the firewall to the engine plate. That ryobi is perfect for narrow cowled designs.
Edwin