Here's my 2p worth on this.....
..... as others have said, beware of ANY replacement crankshaft - the Lotus one you already have is forged and toughened/coated after the bearings are polished to size. Some (good) engine builders will be able to machine them down a size and get them recoated (which isn't a coating, but you know what I mean!) - but unless you have had a major disaster, you won't ever need to do it.
I am yet to see a crank which is damaged beyond reuse (but I'm sure it WILL have happened somewhere) - usually, just a set of new standard-size bearing shells is all you will need. Again, as well as the usual suspects for getting them, any engine builder will be able to buy them - they aren't always model-specific - and they aren't always very dear.
(I paid about £45 for both main and big end shells).
I was lucky in that I got into a 'group buy' on Isuzone forum for a set of forged pistons at a slight oversize - they came from USA and were made by the same company that Robbie the robot uses (and also Phil at SW Lotus, that the ebay link is to). Whilst I got them at a 'group' rate, even getting one set machined in USA was cheaper than SWL (obviously, as they need to make a profit) - and at the time, the $/£ rate was more favourable to us in the UK. I think they came in at £450ish including all shipping, duty, VAT etc - with rings and gudgen pins/clips.
I wanted oversize, but again, unless you've had a disaster, usually it will get away with the OE pistons, new rings and a bore hone.
As for cams - they don't seem to wear too much (again I got a pair many years ago from Lotus aftersales for a stupid price of £2 each because no-one ever needed to buy them).
The cam followers are Isuzu parts, but NOT for the Impulse 1.6 (can't confirm, but common belief is that they were from the 1.8 engine) - I'm sure they wouldn't be hard to source if needed (but probably NOT needed).
If you have a good engine at the moment, but want to future-proof it a bit before it suffers major wear/failure - you may well just need a set of crank shells, piston rings and some work on the valve seats/guides/seals.
I am just collecting the parts together for the day of major failure whilst they are/were available, but i have a spare engine with 130K miles on it that I will re-build with all these bits, swap them over, then refresh the one in the car with (hopefully) just shells and rings. It's a job I've been planning for a number of years, and will get done when I get round to it.
As for the machining, there is a company in Bristol who i have used for many years (Price Brothers at Avonmouth) - I will give them the block/crank and the parts, and let them do the bore machining and fit the bits together for me, so I just have to do the head and rebuild it.