If I'm not mistaken the 04-06 1200 Sportsters use Buell heads already. You can spend the extra money but I don't believe you will see significant improvement over porting your stock heads for the cost. The stock heads also retain smaller valves and when properly ported, increase the velocity. Boring your stock cylinders also has advantages. The dealer would probably like to sell you new cylinders. The cylinders are the same, just bored to different sizes. You can save money and have a better cylinder when done if you bore yours.