SPEAKERS! Use them instead. The risks involved with using an actual chain saw with or without a chain, is still dangerous! (accidents, toxic emissions, etc) The same startle effect can be created using a non-functioning chainsaw, and a high quality chainsaw sound file! Of course, this would only work in a room setup.
Still, it's a safer alternative.
strobe lights, and fog would be added to enhance. In the dark...it's all relative anyway.
You could probably use a
cut-down-to-size pool noodle or any chainsaw-shaped object and still get away with it!
Plus... you're omitting some of the sensory aspects normally associated with a chainsaw "startle"
No longer with the smells of gasoline, oil, and the sound of an idling chainsaw give hints to Trick or Treaters about
that "chainsaw man" around the corner! Without sensory cues, guests are completely clueless!
Ready, to crap their pants in terror!
Here's a sound file I created that you might can use:
http://www.megaupload.com/?d=XOJY48CF