Understand that due to the way the engine is constructed the whole engine, apart from the right hand head and the crankshaft, needs to be torn down to fix this. This is not a 10 minute fix! Nor a cheap one!

Here is a microfisc showing the position of the these gears:

You need to replace #11 and #17 gears. Note that you are replacing 4th and 2nd gears.

23440-371-000

23471-371-000

You should also replace the thrust washer #19.

Also, you will need to carefull inspect the selector forks for wear, as above. Any sign of wear at all is a no-no.

Needless to say, all these parts are now obsoleted by Honda. So it is a matter of hunting down N.O.S. parts or good used ones. The later 78-79 models featured slightly heavier selector forks; worth looking for these if replacing.

Also, realise that it makes sense to go through the engine and renew all the oil seals, timing belts if they have not been changed recently along with their tensioner springs, possibly starter chain, check the water pump, oil pumps and the clutch plates too. Basically just check every wearable thing, as you really don't want to be doing it again!