For deepening the o-ring groove you could probably strip some fine sandpaper if you dont have a file small enough.

If you find your own o-rings investigate the material type so its compatible with the fuel. When I happened upon this problem I considered a silicon o-ring but couldnt find one as fast as I wanted it.

I have taken to running my cylinder to tdc and holding it to let off compression closing the exhaust port for storage. This at the suggestion of Verne Koester (sp?), hopefully to prolong my bearing life. Just an experiment at this point.