the little D series will go forever if they are looked after properly - my D15b7 has done almost 320k kms and it still goes like the day we bought it with no major issues (bought at 150k kms)

if the sound is coming from the upper part of the engine than i would say that the tappets just need adjusting - consider that most D series will develop a bit of valvetrain noise anyway as awaiken suggested

i would get a mechanic to do ur tappet clearances and ur ignition timing

do you know when the timing belt was last replaced?

suggesting a swap or a rebuild from something like this is a bit of an over-reaction at this stage