you can clean the matrix quite easily, removed from the car, buy some water based de ruster, Halfords used to sell it, but you can get it online, its a concentrate you mix about 90% water to, fill the matrix, leave to work over a day or two, repeat if necessary, you will need to then pressure test it and if need be, have it repaired, my personal experience of demistermans matrix units is not good tbh, but just a personal experience, his mk1 escort ones are a diabolical fit, and you hav1e to file out the pipe holes, but the cored area is around half of that of an original unit
as an aside, this is also excellent for cleaning the water channels in heads and blocks on engine re builds, you just need more of it and a tank to submerge them in
I have actually done an entire water system in situ with this method as well, and it does work, however, once done, you need to replace things like the water pump as it will have likely to attacked the seals