I have a 3.0 CF3 project Spider in my garage, the engine has been rebuilt over the last 9 months after the previous owner had cooked the engine letting it run dry of coolant.
I will offer my experience:-
I replaced 3 pistons and one liner and that was the cheap part of the rebuild, a decent complete engine gasket set is near £300 including the silicon gunge for the sump and I advise you to change all bearings in the bottom end, mains and big ends.
I would have concern about damage to the small ends which see all the heat from the pistons when the oil is not performing its cooling job, these could have gone tight or scored without oil and will cause premature failure of your rebuilt engine.
I am doing my project for the love of it, heart over brain. I have used a local machining company for a lot of it and I am lucky I have the background to do the mechanical rebuild myself. otherwise the costs would be in £1000s by now, I am up over £1500 as it is with the head rebuilds.
Common sense would suggest you suffer the change of engine serial number and get a replacement lump with some sort of warranty.
I feel for you I and will take the advise on the oil cooler and feed pipes.
Good luck with whatever you decide to do, we need to try and keep the rare RHD V6 Spiders on the road if we possibly can.