Hi Chris and
First thing - love the Porsche, even the photo is in the "right" setting. I've never even sat in one of those but they look so right for their period.
Ok, another 2 cent's worth on your last question "would an engine swap add or detract from the car ?". Personal views here, but if I were looking for another car I would look very, very hard at any drivetrain modifications. Don't kid yourself, any move away from standard is going to need significant engineering skills and I'm not talking about just welding up brackets or making sure it's all in a straight line. Weight distribution ? suspension rates ? brakes ? Lotus cars are all about bendy roads and you don't want to spoil that aspect.
Mods like Joe and a few others are doing are reasonably well documented (on the Yahoo groups for example) and there are others using Vauxhall & Toyota so you're not trail blazing on your own to sort out issues. In the UK there's Banks Engineering who have produced some stunning cars with modern drivetrains, but the force behind that company is a lifelong enthusiast with a sound engineering background. Have a browse around his website
http://www.banks-europa.co.uk/welcome.htm and you'll even find one using a Brabus engine from a Smart car.
My personal view is that if the project car is a shell, then it's open field and you do whatever you want to; getting a car on the road that brings you pleasure even it it has the "wrong" drivetrain is better than a car that's crushed & lost forever.
But be aware it's more than likely you will never sell it on and cover your costs unless you do everything from scratch and don't count your labor. Even then I would think it's a close call. To me that's no problem as I never try to "justify" or run a financial ruler over such things because my toys are exempt from all common sense, but if you have a significant other who thinks otherwise.......
If your project car is mostly complete then I'd restore what's there, even the Renault engine. It will be the quickest way to a roadworthy car and after you've driven it as originally designed then is the time to decide what needs changing to suit your driving style or modern conditions. You'll appreciate the modifications and have a long term hobby making the changes.
But looking at your original post - "what about nuts & bolts, detailed specifications, originality" then I think you have your answer.... you need an S1 or more likely an S2 project to restore as original and leave the "wow, that pulls well" for the US side of your garage.....
Brian