The hardest part (by far) will be the engine. I would say the best idea would be to find a broken chainsaw/engine and remove the blade, get rid of all the internals etc so you jsut have the plastic shell. (maybe eBay?) After that it would simply be a case of knocking up a piece of wood in the shape of the blade and painting it then attaching it to the plastic housing.
hmm It'l be tough because of the unique shape, Maybe dig around at yard sales etc for the plastic casings from old toasters/electronics etc.. and stick them together into something similar?
A way if you have access to a chainsaw would be to make a paper mache copy, ie covering the body in paper mache then when it dries you have a shell in which you can stick back together (from cutting it in half to remove from the real chainsaw) and then paint up?
You can get fake chainsaws from shops but they are really expensive and mostly look crappy
Yeah if you get the general shape with a few boxes you can then go over the whole thing with paper mache to make it stronger and to look as if it is one piece. You can also put the handle etc on with plastic/cardboard tubing. The trick will be getting parts that look similar eg for that fan on the side of pic above, maybe using a paper plate with segments cut out of it.
retractable is going to be hard/expensive I would guess. Mostly due to retractable part you're going to have to have springs/rubber bands and all that good stuff. Maybe find an old doll (the ones with the cord in the back which gets them to speak) rip out the voicebox and use the cord set up?
One Haloween I removed the blade from my chainsaw and greeted people at the door with a few revs. First time I've ever seen anyone pee themselves. But, yeah, normal chainsaw, blade removed, gassed up for great reactions.
Ha! That reminds me of one Halloween when I sat on my front porch in a Freddy Krueger costume (hat, mask, claw, sweater, the whole shebang) totally statue still, and ended up getting cussed out by a mom who didn't like the fact that I scared the shit out of her 6-year-old! Ah, memories...