To me only method would be to pour in Acid water mixture in some tub and take a small pump connect the hoses to heater core and back to tub and run that pump for sometime. Then neutralize the Acid and flush the heater core with plain water only for sometime....
Pray that it's not rusted internally. If it is then get ready for a dashboard teardown for replacement of heater core after flushing because it will start leaking.
Used heater core for Cultus costs around 1500-2000/-RS and remaining would be labour for opening all the stuff near to 2500/-RS..