Take it from me just replace the gears my bike did what you're describing after I bought it (in 4th/5th though) gunned it one day and the gearbox siezed locking the back wheel at 130mph
It was a simple job for me without a manual with one or 2 sticking points other than that a screwdriver and socket set will do more or less everything, as I say thouugh I found it simple you may not and thats fine but dont force it everything you're gonna be working with should be really easy to get in/out with minimal force if you REALLY get stuck ask someone for help cus it can go from simple for me to nightmare for a mechanic real quick and its not worth riding on a botch job.
The real pain for me was the clutch basket retaining nut, it tightens from being under load so when you get to it if you dont have a clutch basket tool you're gonna have a really hard time getting it out we tried everything and ended up cutting it off in the end but with the clutch tool a decent breaker bar and a 25mm? (i think) socket will do.
Other than that once you've got the clutch basket out go ahead and take the engine out (I found it easier to semi strip it down while it was on the bike) turn it over and there are some big bolts on the bottom once you've taken them out and split the case (shouldn't really take much tension dont be like me and wedge it with a screwdriver could be you missed a bolt just tap the sides with rubber mallet to get it to realease) the gearbox is right there pay attention to how it is orientated take the 2 mounting shafts for the selector forks out and the gearbox should basically fall out.
Go slow and take pictures I would strongly advise that if you go for a complete box make sure to get new shifter forks as they tend to wear out you may as well replace them (mine were so worn they had jagged edges and one of of them had a piece of the fork broken off) while you're there and give the selector drum a good look make sure the tracks dont have rounded or stamped edges otherwise that will ruin your new box.
While its open give everything a once over for wear cant hurt and may show up some other little gremlins waiting to pop up, I'm unsure if its nessisary but its a good idea to make sure that the gearbox is in neutral when you put it back together so it turns freely and everything rests in place.
Also make sure you use silicone sealant around the edges when you put it back together except on the big bearings (you'll know it when you see it) otherwise its gonna be leak city.
You'll likely need help to get the engine back into the bike, after 4 hours failing to get it on my own the way the manual tells you to I layed the bike on an old mattress and went in sideways with my dad guiding it job done.
As I said before as long as you take it slow pay attention and dont force anything (clutchbasket nut again is a pain in the neck but if you put a complete box in you're replacing it anyway long as you dont damage the basket go wild) you should be just fine, but dont half-ass it or cheap out if I were you it would be new gears new forks new drum (or within reasonable condition from a breaker) and remember to break it in slow the gearbox has to wear to the engine a bit first.
hope this helps