You might be interested to know that your CF3 Twinspark has the weakness of oil control rings that don't control the oil - leaving it on the cylinder walls - hence the high oil consumption (it isn't due to bore wear) - and this fault will be shared with many 147/later 156 engines, the JTS 2.0, and even the FIAT Stilo 2.4 5-cylinder.
These engines all have a 2mm-wide oil ring with tiny round drain holes. These block easily, which is why regular oil changes and full detergent oil are important. I have machined pistons to accept the 3mm-wide oil rings (with drain slots) of the earlier Twinspark. Also, going back to the earlier style ring set, the top compression ring goes out from 1.2 to 1.5mm and the material changes to chrome molybdenum rather than cheaper cast iron. This in spite of the fact that the earlier Twinspark ring set costs about half what the later Twinspark/JTS ring set costs.
Result is a huge reduction in oil consumption (to one-eighth of the previous level, on my 156 and another 156, to about a third for a 147, and to one quarter for the Stilo) and attendant advantages in reduced carbon buildup and improved spark plug condition. Combine with a slight skim of the cylinder head, the high compression of those new (old) top rings, and a new Variator, and you can expect full original power to be restored.
There is also no need for the thicker 10W60 oil that some people (including FIAT/Alfa) recommend as a crutch to reduce the oil consumption. That would be the case if the engine were worked hard in a hot climate and if the consumption were due to wear. Actually, my experience is that a thinner oil (5W30) drains better through the original rings - the thicker oil makes the consumption worse instead of better!
So all in all, while a 147 engine will drop in and work, I feel you should rebuild your original.
It takes a couple of days and the engine doesn't have to come out. There is a school of thought that says cylinders don't have to be honed (original honing marks will still be there), but I do it anyway with the three-stone hone just to give the new rings the best chance to bed in. This is achieved by driving the engine pretty hard from the get-go - 'running in' for the first 1000km means a cheap mineral oil, no short trips allowed, and revs kept up.
I'm not sure what to suggest with the gearbox - I suspect the GTV TS box resembles a FIAT Tipo or Alfa 155 box as those have similar rod changes - I don't know enough about the internals to say what will interchange with the 147/156. I think you have the right idea to swap the box complete, as it would be more difficult to rebuild the gearbox with a satisfactory outcome than it is to rebuild the engine. Of course, put in some new oil and give it a bottle of shift additive in the meantime, if you haven't already