It is hard to describe just how bad the vibrations were, but my main concern would be the engine breaking loose from its mount, or the prop from the engine; which is a sure recipe for disaster. I would not be bent on getting the engine to run correctly, but rather run it at idle or even switch it off; or use the engine only inasmuch as needed to reach a suitable field. Then again, I fly a fairly light machine (450 kg ultralight) and have been taught to consider an outlanding as a non-event, only slightly more dangerous than any other landing. If I flew a PC-12 (just to name an example) things would be quite different.