Help!! Fors Sierra Sappire Gearbox

[Saberhunter]

Good Day

I was wondering if someone could help. my 2 L Sapphire has a type 9, 5 speed gearbox
About a week ago the fifth gear stopped engaging. Two days later the gearbox started whining. So i found out: to get it repaired would cost more than to buy a second hand one.
I can get the 4 speed bullet box but i don't know if it would fit. Any Ideas?
Thanks
Saber!

[Saberhunter]

So i bought the box.
Surprise, surprise... it didn't fit. Out of desperation with no knowledge of gearboxes whatsoever, I started opening up the 5 speed. The little pin that engages the fifth gear fell out. Is there a way to secure the pin more tightly? Maybe some sort of Epoxy? I'm planning on fitting the box this weekend so any suggestions would help.

[Saberhunter]

Okay new problem

I fixed the box and put it back

now when i release the clutch (in any gear and neutral) there's a sort of whirring sound. I replaced the Thrust bearing but now i have this noise

Thanks in advance
Saber!