The input shaft for a later light or no flywheel bike is shorter. The trans can be opened up and the suitable short drive shaft substituted for the long one. Of course you will have to swap over all the gears. If both drive shafts have gears it may be possible to do it by a complete swap but there was another change in I believe March of '82 when the three gears with angle cut teeth had the angle changed from 15 to 17.5 degrees. You can not mix these gears up and can use either set up but it has to all match.
Never memorize something you can look up.