Originally, your motor had pistons with a shallow dish in them. The compression ratio was 8.0:1. US-spec engines had pistons with a larger dish, and a 7.6:1 compression ratio. The Bus pistons had a much larger dish...
There are flat-top 8:1 pistons available now from several sources; I think Mahle makes some. Not all that cheap, sadly...
A lot of suppliers in the US can get the parts for you, but shipping and duty and such is going to make it more expensive. You can probably check the German retailers like Mittlemotor and Reichert and so on. Maybe LA Performance can get a decent deal on them??
For more compression, you can go with custom pistons or you can flycut the heads. You can do other stuff as well, but those seem to be the most popular options.
Measure everything (combustion chamber volumes, deck height, dish or dome volume if any, etc.) and crunch the numbers to see what your compression ratio will be. Small changes in volume can have large effects on the ratio.
You can go larger on the pistons--the 96mm ones are a pretty popular option, and will give you 2056cc displacement. To figure the compression with them, you will have to mock up the motor and do the measurements and crunch the numbers. Not sure who you'd get 96es from in Europe, though. There's a place called AA Automotive in San Francisco that carries them for something like $300, but the cylinders may be a bit suspect. It might prove better to have your stock 94s honed out to 96es by someone who knows what they're doing, then use the aftermarket 96mm pistons.
--DD