Kev,
With Mainline out of production for 20 years or thereabouts, any spares you do find may well be nearing end of life themselves. The gears have been a consistent weakness in this product range. That said I would be looking for commonality with gears on other Mainline product like the class 45, and maybe in some of the earlier Bachmann products, simply to widen the area of search.
The other alternative as you probably realise is to go for a replacement Bachmann chassis. Bach do sometimes have chassis for sale, and have offered good prices on repaired returns sold directly at the Warley show. Inconveniently for you, there probably won't be any Warship chassis right now though, as it has been some time since a run of Warhips were sold. Very much a matter of being ready to pounce the moment the right item is on offer.