Engine controls P323
I would bet that it is the cable linkage. If it is old it is probably corroded and needs replacing. You can test this by using your hand to move the shifting arm on top of the transmission. It should be easy but firm to shift from neutral to forward and reverse using your hand. If not, need to check the transmission. Is it full of oil? Does the oil have crud in it? If so you can take the top off the transmission and inspect the shifting forks.If you need to change the control cable it is not too difficult.