Hi,
We've just gone through the same dilemma as well. We had a really ugly Glow-Worm in our sitting room, however it was in full working order so we couldn't just rip it out. Its installation date goes back to 1972! What our local plumbers told us, is that such old units are still being serviced BUT not manufactured nor installed any more. Basically, you either keep it the way it is or replace the whole house heating system and get it ripped out.
Depending on the type and size of property and the model of your new heater the total cost should be around �2500-�3500 to do the job, depending on where you live etc.
My strong advice would be to get it replaced with energy efficient modern heater - is it will free up lots of space (that massive hot water tank will have to go!) and you can get nice reclaimed fireplace put in where the ugly monstrosity used to be.
I hope this helps.