How can I repair(or replace) a 20 year old boiler?

My Potterton Kingfisher 2 flu boiler has suffered from a corroded fire chamber and I have been told that parts are not available for this boiler. I would like to replace it with a new boiler in the same situation as the old one (in the cellar) in order to
avoid all the piping being altered.
How could I do this? Are there any whole new or used same boilers available?