Bob,

Most glass filled nylon engine mounts cost only $3-$6, and if your mis-drilled mount is of this type, it is best to discard it and get a new one.
If it is an aluminium engine mount, a good inspection can tell if it is ruined.
When buying such an aluminium mount ($14-$40) it would be best to get one that is already pre-drilled for your particular engine. Another possibility is to get a hole locator, like [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXK378&P=7]this one[/link].

Another possibility, is to get a mount that has slots instead of holes, or a "press down" bar, that can be used to mount just about any engine.

Vibration damping mounts do isolate your model from the engine vibrations, but the fact that the engine is allowed to vibrate more freely, could end up in broken headers, exhaust stacks and even engine mounting lugs.

I would keep away from most of them, especially with two-stroke engines.