[Hanuman]

I'm actually of the opinion that you use which ever engine gets you up and running with the least distraction.
If it were a restoration for sentimental or investment reasons alone, then originality is important. But if you want use it, then that's the main focus.

With 2 engines available, it takes the pressure off, somewhat. If the spare engine will go in with the minimum of fuss, great.

It might be worth having a closer look at the existing engine.
Long term poor storage of aluminium heads- porosity can lead to coolant leaks. Both engines may be at risk.
Valve guides,and especially seals would probably benefit from attention.

The original engine is opened up. How are the bores?