The smallest gasoline engine i have seen is 25 cc and they fit quite nicely in 1/5 scale but it will not fit in your truck.
As far as I know smaller engines are not made as the 25cc is taken from small gardening equipment. Although it might be worth asking the aeroplane lot about the engines there are some huge planes out there.
I dont belive that a petrol engine would really be practicle as they have too much torque and not enough rpm to be useful. Let us know how you get on please