1972 350SL - Classic value engine swap dilemma

[Hood]

I have a 1972 350SL which has been rebadged and has a 450 engine in it. Alas the motor is knackered so I am intending to replace it. Would it be better for the cars classic value to return it to it's original configuration? If so, is it a straight swap? I was told some ancillary parts like the exhaust manifold might need replacing. This is going to be an expensive piece of work so I want to make sure I am doing the best thing.

 

[Hood]

And a further question!

Should I get a second hand engine for £500 (with approx 80k on the clock), or a remanufactured for £2000? It is currently without a service history so would the latter improve it's resale value in the future?