The Fiesta motor is based on the crossflow block, which is slightly taller than the pre-crossflow block used for the Lotus TC motor. Nevertheless, people have put TC heads on the newer block with a spacer for the front cover, or you can get specially cast front covers for them. As also mentioned, you can get newly cast blocks and heads, and in truth I don't think they are really pricey, at least not when compared with other makes. I believe that in England you can buy a brand-new Big Valve head, complete and assembled with valve seats, guides, valves, springs, cam followers, camshafts etc. for about 4,000 pounds (say $6,500 USD). A lot, maybe, but not exorbitant, particularly when you look how the prices of Elans are escalating.
The diff is 100% Ford in a Lotus casting.
Front Suspension is based on Triumph Herald / Spitfire; brakes are model-dependent and may be Triumph or Ford Cortina; engine and transmission (apart from the head and diff casing) are Ford Cortina and its kin, rear suspension is Lotus, as is body and chassis frame, electrics by Sir Joseph, trim from a scavenge of other makers' parts bins!
Much though I enjoy this forum, excellent support for Elan affairs can also be found at
LotusElan.net which has a good cross-section of Elan owners from the UK, US, and other far-flung places and is, like this, a pleasurable forum to visit.
I have no connection, by the way, and hope Basil doesn't mind me plugging them.