Ok the Opel 6 cyl is similar to the Missfire (aka Starfire) 4 cyl. The 4cyl is basically missing 2 cyl. So you should be able to source a few parts through that avenue.
Now make sure your sitting down
bad news on the head gasket front If your mechanic kept the old one it may make life easier you can get say a copper one copy cut or machined up your going to need to source one from the UK.
A complete engine gasket set may end up costing you around $1000 by the time it has been shipped from the UK to you.
My suggestion to you is start looking for some Opel forums on the net, someone might be kind enough to help you contact a parts dealer in the UK or even ship you the parts needed.
do I hear engine conversion??
best of luck
Scott