Help With 5 speed

Hey guys. I need to rebuild my gearbox and i've been searching for some articles about this without success. i was just wondering if anyone had a link or advice.
It is a steel case 5 speed(not sure what model it is out of as it is in a v8 1Tonner)and i have lost 1st, 2nd and 3rd gears. it rattled alot then went bang.i realise it is probably cheaper and easier to replace the box but it is now a project to rebuild the entire car.

Any information will be greatly appreciated

ty
Feral