Whoa! Don't go out and order those 350 Chevy pistons just yet! The 350 Chevy has a 4.00 inch bore, the Opel is only 3.66 inches. I think the pistons you are thinking of are the Chevy 265 pistons. This engine was produced from 1955-1957. The bore is 3.75 inches, or .090 more than a 1.9 Opel. They are flat-top piston, and they have a piston pin that is .927 instead of .906 (stock Opel), so the Opel rods have to be opened up to fit. You also need to bore the block .090, and get yourself a 2.0 litre head gasket. That's about it.
For a camshaft, I think you would be looking for something with about 225 - 230 degree duration @ .050. This will be strongest from 2000-6000. I prefer to use 108 degree lobe separation for better torque, but it will idle a little rougher. Most people use 110 degree lobe separation, it idles smoother but has less mid-range *oomph*.
Opels are not cheap cars to modify, at least not anymore (the '70's were another story). We tend to forget they ARE German cars ($$$$), and they are sort of bastard cars in this country, so parts are not readily available everywhere. Opel GT Source does a great job of not only stocking these hard to find parts, but also reproduces a great deal of parts. Some other guy that sells parts for multiple makes but carries a few Opel parts may save you five bucks here and there, but really doesn't give a damn about you or your car in the long run. I try to support the guys who are supporting US.
As far as other parts sources, there's Opel GT Source, as has been mentioned, Opels Unlimited, Goin Manta, TGSI for performance parts, Orphan Imports, and a few others I can't recall offhand. In the past 10 years, at least 10-15 other business' specializing in Opels have gone out of business. Not using the existing companies may contribute to their demise, and the day you need a specialized part the local store can't get, you'll be out of luck!
Well, enough of my lecturing, I didn't mean to turn this into a documentary on "Saving the Opel";.
