You could probably get a schaller 456 in there without it looking too much like theres a gaping hole in the guitar. They're about the size of an FR and the pegs, if the trem has 8mm pegs, should go in the same holes without much/any trouble.
Failing that, I'd say just tremolno it. Screw the claw screws into the body a fair bit so the trem sinks into the body, lift it to the correct level, lock the TN up and tune up and you'll have a fixed bridge that makes the guitar more resonant and lively with more low modes as well (vibration transmiting through the TN into the body).
