Disclaimer - I know next to nothing about FPGAs.
I don't see why you would need a huge FPGA to emulate a PowerPC. The instruction set shouldn't be harder to implement I'd think (after all, PowerPC is supposedly RISC). I think all those extra transistors are in the PowerPC because its a more modern, powerful chip than a 680x0. Those extra transistors are doing things like implementing multiple units and stuff. A modern Intel CPU shares the same core instruction set as a 386 but obviously use far more transistors.
Anyway, I do think a PowerPC in FPGA would be a bit silly unless there is some great need to run PowerPC code. Whether you implement 680x0 ISA or PowerPC ISA you are probably going to get about the same performance on a given FPGA. The only advantage I see to PowerPC is that modern variants of the ISA would include SIMD, which might help the FPGA state instructions to the CPU in a way that helps it process data more efficiently.
