It's a good point about 'availability' of parts - and that is why it is good to go with someone who you feel is 'good enough' and that only comes from talking to them, and seeing their work.
Whilst Simon paints a bleak picture of parts that you can get, it is important to note that it isn't often a problem, as long as things are done right!
First, the lack of oversize bearings isn't a big problem as I have NEVER seen an Elan crank that needs a grind - they had such a good nitriding/hardening at the production stage that the actual crank journals just don't wear (I'm sure someone will jump up with an example of a catastrophic failure over the past 30 years, but I can't remember one).... it is due to this hardening that the crank shouldn't be re-ground (it will remove the hardening) - but I suspect that if it WERE done, and appropriate Main/Big End bearing shells were used, it would be fine.... but it won't last for 200K+ miles!
The big point is the oil holes in the shells - unless you read the Manual (or know the contents), it is easy to look in a manufacturers catalogue and buy parts which will physically fit, and work fine..... for now. Without the extra hole, oil pressure will be higher than expected, as it can't send a squirt of oil to the underside of the piston skirt, so the piston gets hotter.
Realistically, none of these things will matter if you drive down to Tesco every Saturday, and take a drive out on a Sunday....... and rarely go over 5000 revs for sustained periods. But if you are taking it to the red line on a track regularly, it will end in tears.
One further thought - if your engine needs new parts, make sure that the right ones are fitted - we have mentioned the bearing shells, but the actual crank is different between the Turbo Elan and the NA (I don't know which was used by the Isuzu!) to allow for the extra oil hole, and also one was cast whilst one was forged (forged being much better).
The Pistons are different between Turbo/NA, as are the Head Gaskets, Camshafts and variety of other parts/ancillaries - the clutch and the oil pumps are different.
If it were me being asked to do the work (and I'm NOT offering!!!!) I would strip it all down and start again - and I suspect anyone who is worthy of the job would want to do the same.