I have never thrown a leg over a Guzzi, but I like the look of the engine.
Having converted a 650 Yamaha street bike to a dirt bike, I can say it is better to get what you need for the riding intended, than to try to make do. On the other hand I had a lot of fun with that bike even if it did land like a ton of bricks.
The 1800 VTX is a sorry dirt bike too. I know first hand, having taken it places it should never go.