I would drain it all down and replace it with water less coolant from Evans. You have to use a Prep fluid first, this I believe can be used again on other engines. But once you use the Evans, it last the life time of the engine. It might be idea to repace the pipework if required. I have no connection with the company, just very happy with it. Look at
https://evanscoolant.co.uk for more info.
