Is it ok to fit a wood-burning cette with pipe of 18cm into chimney 120cm wide?

you can fit one, but you have to close the gap between the flue pipe and the chimney at its narrowest point with fire cement and asbestos rope. TYou can only do this by knocking out the chimney above the wood burner, then repair it again. it is the same work as installing the old Parkray boilers. Unless you do this, you will lose any draw on the chimney, and have wood smoke coming into the room,