I think you mean a shaft coupling of which there are many types suitable for directly connecting an engine to a generator, the best and most expensive is probably a tyre type, a sub type of these is where one part bolts directly to the flywheel and the centre of it has a taper lock type fitting which grips the generator shaft, these couplings allow closer positioning of the driving and driven parts and will cope with quite a bit of misalignment but with any job like this its always best to get it as close to true as possible.
Mark.