You may need to make the gasket, not hard but use Cork....
The copper washer you want, find a Bosche dealer that rebuilds injection pumps, the one I deal with here in Sudbury seems to be able to fabricate or find anypart even stuff that isn't Bosche.
Check your decompressor handle for an O ring that fits in the groove near the handle end. If its Missing thats a Cat part ( Or rather I found a perfect match in CAT O ring spares kit. But its probably a standard size ring. )
I don't know how big a mini Petter rocker box cover gasket is but I have a spare for big Petter. Post rough guess measurements and I'll see If it the same and Maybe I can copy it or part with it....
Doug