I did that last year for our cakewalk at our church's fall festival. It turned out great! I also did a glaze...can't remember the exact recipe, but it was cream cheese, pwd sugar, lemon juice, milk, i think...then I just used orange and a little yellow coloring to get the "pumpkin" color. Put it on a cooling rack which was on top of a sheet pan and poured the glaze over it. It coated really well and looked even more like a pumpkin the drier the glaze got. And since it was a glaze, the form of the bundt pan still came through really well - a frosting would have covered it up too much. Fondant stem and curly-q vines were added to it. I was actually a little impressed with myself at how realistic is was...lol
And it really was super simple. Oh - I used spice cake too....yummy.