I like your thinking about runninig Dual trx250r except multi engines are only allowed in the streamliner class and the large motor cycle classes. I would really try to see if you could find something in a 500 cc two stroke, Bonneville is all about horse power and a good two stroke will make more than a four stroke of the same displacement. I would look at some of the snowmobile engines as some of them are big power. Another thought would be the Yamaha twin cylinder, water cooled, two stroke that they ran in one of their ATV several years ago, I have hear some pretty big numbers for them also. Brant and Speranza ran a small Honda four cylinder (destroked 600 I think) and they are pretty good engine builders. The record is 202 so you need a good car with a good shape and some pretty reasonable hp, probably 125 +.
Rex