Good stuff.
Making the missing part wouldn't be too difficult - stretched sprue and a piece of plastic card. Plastic strip, card or sprue cold form the mounting points on the rear.
That said, which seat is more accurate for your taste ?
The resin seat, though nice, and with the bonus of the seat harness, lacks the control column bracket, but the kit seat looks rather too "angular", even with the subtle curve to the top, compared to photos and drawings of the real seat.
I think I'd make the relatively small effort to add the needed bits to the resin seat - but check that it will fit first !