Gearbox information

Recently i bought a 1981 pantah 500 sl and have inherited gearbox problems. I have read many topics, but still don’t know if all gearboxes are interchangeable, has any one on the forum any knowledge that you can inform me of any parts that are interchangeable. The full strip down will be completed on Thursday of this week and i’m hoping that i will be able to find the causes of a lost 3rd, 4th and 5th gear. If in the meantime i can learn of any pit falls that may arise, i will be most grateful.