Welcome to the world of grown up MGs
(my wife has a ZR)
I would check what coolant is in the engine, I understand mixing OAT and non oat can cause sludge to form.
Is yours a hatch or a saloon, they have different badges if you are replacing the whole thing, the inserts are the same 59mm as the mk1 ZR.
Are you bring it to Zed10 on Sunday?
James