Paeco, out of Birmington Alabama, used to work on Opels some, and still have the tooling to do it. Expect to spend @$750 for ported, polished, milling, oversized valves and springs, andthe whole works assembled.
They also sell a stage 2 engine they claim is good for 152Hp for $4700. ($3900 if you assemble it yourself.)
Their # is 205-823-7278, or you can email at [email protected]. I have experienced their work on a fes old Datsuns and MG's and been impressed, but I've never been able to afford their work myself. Their catalog also lists a 6 or 9# aluminum flywheel for $470 plus $129 for the ring gear and their header for $210, for a rough cost comparison.
I was on the phone with Roger last week discussing the mods he did to my "New" Opel Senator. The phone number (Tel: (XXX)-860-6550) is correct but the area code has changed. I'll post the correct one when I get home.