Hooked, I'm suprised your water
pump was so expensive, Perkins parts
are normally very reasonably priced. The closed circuit(engine) water pump can be sourced from parts4engines.com
(+ other Perkins parts) the seawater pump is a Jabsco
Most Perkins engine parts
, except marinisation parts, can be sourced from a local Massey Ferguson tractor dealer, they used the same engines. Marinisation parts can be obtained from Bowman.co.uk
I'd be inclined to manufacture a new exhaust elbow from mild steel, or (if you are going to keep the boat), stainless 316 grade only, with an extra 'wear' plate welded inside where the water hit the casing, assemble with bolts coated with copperslip.
In respect of the corrosion I would strongly suspect/test your Cathodic protection for a fault, either none fitted, wire connection broken, sacrificial anode eaten away.
I hope this helps.