The bike you are describing is called a Derny.
The guard on the rear wheel is to prevent contact with the bicycle wheels drafting it.
They can be purchased here: Derny Sport UK
They are a bit pricey but they are known to be very reliable. One thing to note is that the engines are about 98cc. This should give you some indication as to what you need for power. I would be looking for something in the 6hp range.
If I was to build a bike for your application I would chose a Morrini motor or one of the larger rear rackmount kits. The Chinese kits are innexpensive but your application would be running it WOT the entire time and I am not confident that it will hold up to that in the long term. I have noticed that most Derny style racing in North America seems to use a small motorcycle or a moped for the Derny.