The US got the 1.4/1.6L motor from the Brazilian Opela version of the T-chassis. It's really not an all-together bad little lump, and if I had one with a 5-speed I'd even consider it a good Locost choice. I've seen a few of them swapped into Opels over the years, and it's a good fit in a GT if you aren't looking to add any power but want a little more room around the motor to work on it and want easier to get parts. It's an all iron motor, not aluminum like the Vega, and if you factor in how we all abused any Chevette we got into in high school I'd consider the motor pretty robust.
The diesel Chevettes got the same little Isuzu diesels as the LUV and I-Mark, and IIRC it has the same bolt pattern as the 2.2L motor Isuzu used ineverything from the I-mark up to the Trooper. That motor was available as a real screamer in the turbo Impulse, and should be an easy swap with almost no fabrication required. Of course you might as well start with an Impulse for the Lotus handling bits, unless you're looking for a sleeper. Keeping the diesel and adding a turbocharger might be a viable option too, if you're into making exhaust smoke with your tire smoke.
The UK version Chevette that was the real screamer had the Vauxhall slant-4 in it, and that motor just loved aftermarket induction and tuning. I've heard of guys with bolt-ons getting easy 150hp+ and being almost untouchable on the curves too.