If islamic finance is implemented, what will be the role of Central Bank ?

if you are talking about islamic finance which is used in the modern world system then there is no difference between islamic and commercial banking both are the same things..the modern islamic banking is actually the backdoor for commercial banking

if you are talking about the real islamic finance then it is not possible while using the paper money or paper note ...

Islamic finance is just a way to grape more customers, I was able to check on banking for lots of banks, calculations are the same on all banks, loan is a loan but called finance on Islamic Banking.
at the end I really recommend that you choose what best fits with you as from Islamic point of view there are some saying that it is ok to use it.
otherwise you can do as I did for my self, (Never have a loan from bank) you can have a deal with supplier directly to make payments on installments outside the bank.
for the second part of your question, Central bank puts the same rules for all banks so how we can consider some of them Islamic and others not? they put it with minimum and maximum profit rate (consider the word Profit rate as they some times call it Morabha)

but their main role is to control monetary policy.
How would they control interest rates ?