Oh dear, you may have opened a can of worms here!
The Roadster has been built with so many engines now it's kinda down to choice. Theres everything from the standard Pinto as per the book up to a V6 thats been shoehorned in!
Whatever's chgosen will require variations to the design, and I don't think anyone on here will be able to give you a definite: this one is better than any other because... (Except maybe the V6
)
I'm using a 2l DOHC cos thats what my donor came with. This is the usual way of working, so you'll probably get a few reccomendations for the mx5 engine now too.
I haven't really helped much have I?