For the European market these guys are very good. They can build you a fantastic sounding engine.

http://www.johnmaherracing.co.uk/index.html

In California these guys are very good and they do ship worldwide: www.serranosvw.com

Here is a sample pricing of their work:
http://www.thesamba.com/vw/classifieds/detail.php?id=342467

As for speccing a hot VW engine, the sky is the limit and they can be configured in so many ways that its mind boggling. For some basic technical articles checkout

For street use in a replica with relatively narrow tires I think 175hp is really more than plenty. The car doesnt weigh much and likely you are not going to attempt any land speed records.

I cant spec a engine build for you here, it is quite a involved list of machine work and parts selection. My advice is to gather what knowledge you can about parts and current state of the art VW engine builds and then select a reputable engine builder with a national or international reputation.

There are forums for VW powered replica Porsches and hot VW's. It is likely someone from the EU is there with current and practical knowledge of homoglation issues for the Continent.

What you also need is some practical advice regarding transmission gearing/engine output/tire size for those cars.
That will vary depending if you go with a 4spd or 5spd box.
For that you will need input from some who has been there and done that with the replica of your choice.

And now for some sacrilige.. The replica you build will be better performing and more reliable than any 4cyl Porsche speedster ever was when they were new! So enjoy it wholeheartedly!