Perkins phaser full of rust

i got a call today to look at a manitou 629 that sometimes overheats it is a 4 cylinder turbo perkins phaser. the owner got the radiator reconditioned last year so thats fine. I removed the water pump to check the impellor it was fine but very rusty. all inside the engine was rusty there is even rust stuck to the main rad hoses internally. there is a small steel hose probably about 1 inch outside diamater coming from the top of the water pump heading towards a cooler and it is nearly blocked solid inside! has anybody else ever came across this problem or any idea how to sort it? it runs fine most of the time but cannot do any heavy work without getting hot.

Perkins phaser full of rust