how many parts can i put into a boiler

[stevetheplumber]

tricky question in my loft i have a o/f vailant boiler which is over 20 years old a few years ago i bought another onebrand new exactly the same now im only allowed to fit condensing boilers so obviously i cant now fit this boiler but how much of the original has to remain ? i would assume at least the chassis

 

[Blake]

As many parts as you like to keep it running