I'm pretty sure two 777 radials will fly a 50 pound plane with out great difficulty. It's a bomber not a fighter for starters. So speed isn't important. If you use the calculus of 1 hp per 10 pounds you end up needing 5 hp of thrust.
I have see the 777 fly a FW 190 that was 30 plus pound. It was decent performance. Could it have been better? For a fighter maybe yes. But it was pretty scale looking.