Thanks for your help! Have manège to remove and replace the drive gear now. Applied plenty of WD40, and got it to rotate with some stilsons then applied more WD40, left it for three days and had at it with some mole grips and finally managed to pull it out!
Have used emery paper on the shaft of the new one to ease the fit a little!
Thanks for tip about kick start stop - have ordered a new one and will fit that before reassembling the gearbox.