Hi Sid, Welcome to the Wonderful World of Spyders!
I have a CB Performance 2165cc VW Type 1 motor that put out 186 peak HP on the engine dyno, and IIRC about 150 lb/ft torque. Or there abouts... The whole car weighs 1408 ready to go on the track so it scoots along quite nicely.
As to what motor SHOULD be in these...the 64 million dollar question. My first choice would be the Porsche 4 cam motor. But they are a little pricey, and rare, and finicky, and not really practical from any viewpoint. A lot of people use VW Type 1's. And a lot of people use VW Type 4's (my next motor BTW). Still others use Porsche 914 motors, 911 motors, VW/Audi 4 cylinder water cooled, Subaru motors, Alfa motors, etc. From this you can see that most cars have a flat 4 configuration, with a few going to 6 cylinders, and a few outlyers with a straight 4 config. Oh, and one crazy (a GOOD crazy mind you) man we all know about put a Northstar in his!!!.
A lot of your choice depends on what you are going to do with the car. I chose my configuration because I didn't know any better at the time I specified the build and my builder recommended that motor. If I did it again I would go with a Type 4 motor, mostly because its a more robust motor with higher torque output. I wanted a car that I could split between fast drives on the street and track day duty. It had to have street manners but still be a blast on the track. I'm very happy with where I ended up and have no complaints.
Any of the above motors will get you a pretty quick ride with the weight so low on the car. You could weigh in how "True" to the original you want to be. A 4 cam motor being 100%, then 914, then vw, tehn Subie, then...you get the idea.
Also think about complexity: air cooled vs. water cooled makes a big difference in all of the "stuff" you have to hide in a very small package.
Lots to think about.....