you need a de ruster, not a de scaler, safetykleen, Halfords and frosts restoration all do a water based de ruster, not cheap, but tends to be very good, I can vouch for the safetykleen one, but not with engine in situ, I did it with block and head submerged in a tank, mine was very heavily rusted, to the point water couldn't flow, 3 days submerged and it was like new inside, and removed all the corrosion even from where you would never get otherwise
whatever you use in situ, you run the risk of opening up holes and making leaks appear where the corrosion has sealed the holes, but at least you can repair them I suppose, then know your ok to drive a distance
if your going to do in situ, I would remove the stat first, once rid of air locks, just let it sit, the safetykleen one will come out as clear liquid once its done its job, you wont see any rust or rusty water when you drain it, if you do, then it hasn't finished, not used the other 2 to comment
I used this on a lot of parts where sand blasting would have damaged the part