The following pictures show to assemble and install the gearbox of the Rotax 123 step by step(without disassembling the shafts).
The installation is quite easy if you know the correct sequence. Everything is done without raw force. You may have to knock the gearbox output shaft with a soft-face hammer through the bearing with the shaft seal. Inserting the shift forks and threading them into the shift drum should also be possible without any hammering.
To install the shift drum, simply push the spring mechanisms on the shift shaft to the side. When installing the bolt for the shift forks, you have to fiddle and wobble a little until everything is in the right position, but force will be never required, under no circumstances.
It must be possible to turn the gearshift drum by hand with little effort when turning one of the transmission shafts and shifting the gears cleanly. Shifting via the shift shaft must also be possible, here you can attach the joint piece to which the rod to the shift lever is attached. It must be possible to turn the joint piece with a somewhat stronger wrist.