It has been my personal experience with gearboxes that the cost of parts is usually prohibitive
Unless it is a cheap off the shelf bearing or Two that is knackered it is usually, way cheaper to buy a recon unit or find a decent second hand one
I priced up some major components for a Capri gearbox a few years back. The total cost of the parts ( Ford dealer prices ) Came to more than the price of a brand new unit, direct from Ford
