The seat pans are a bit different, but the difference *might* be able to adjusted somehow if you're working with a plain metal seat pan. The "normal" R65 seat's tail portion where it meets up with the rear cowl goes upward more quickly and at a steeper angle. The LS seat has a slower, more gradual rise. I don't have a non-LS seat with which to compare to see if the total end-to-end length is exactly the same or not.
If the LS seat is longer by a bit, one *might* be able to adapt a "normal" R65 seat to fit an LS tailpiece by bending the top rear portion downward - essentially lengthening it. But, I don't know for sure. :-/