2.125"? Or 2.100"?
So 2.125" down to 1.850" = .275" you can not get it all but .250" should clean up back to round.
So your stroke gain would be .500".
4.500" stroke but 1.85 on 4" crank will have no Journal overlap.
Anything you offset will be 2 times the stroke gain. So .050" offset will give you .100" gain in stroke.