My mates and I took a run at this.
Costs spiraled out of control trying to keep our engine running. The cost of us building our own frame was also going to be silly, seeing that we could have purchased one for the same price, and saved a bunch of time.
But hey, when we finally got our donor lawnmower to run, it was way fun, despite topping out at 6 mph. I can't imagine how awesome it would be with a proper transmission on a smaller frame. Props to you guys!