You'd need an x86 emulator ported to ARM, as pointed out. Then the iPad would need enough CPU speed (1Ghz ARM? -maybe- it'll work) and RAM (256MB? Possible). Making it playable would also involve getting the GPU calls mapped to the iPad's GPU, which means the emulator would have to be specialized.
It -might- be possible, but it would be a lot of work, and it would probably involve piggy-backing on a project to port some other more popular program. New client is also more feasible, as we already know how the client works anyway.
