If you go to JEGS or Summit automotive they have fans that can be mounted to the outside of the radiator. Most use a nylon strap that goes through the cooling fins so there is no real mounting issue.
I have never checked the temps on mine but if the fan does kick on (rarely) it seems to bring it down pretty quickly. Check your coolant level, and check the connectors and wires to your stock fan. It appears that if you get an air bubble in the Rocket's cooling system, its a little hard to bleed out. Check the radiator itself for bugs, dirt, litter and vegetation. I had a guy in a truck who's radiator was plugged with some kind of grass seed. He said he's never driven anywhere but the hiway, but the seeds must have been blowing across the road.
Good points all Tom. My coolant level is fine, currently running Engine Ice and bled the air out very carefully. I regularly remove the screen and clean the radiator with a water and a soft brush and blow it out from the back. In the winter (temp in the 70's) the bike is fine. When it warms up in the summer (temp 110+) the bike will run at over 200 running at 65. Any stops will bring the temps quickly to 218 where the fan kicks on. After the fan comes on the temps will continue up to 222-224 before starting to drop.
Now, I understand those temperatures aren't dangerous, and in a pressurized system, nowhere near boiling. I'd just like to see the Rocket behave a bit more like my Chevy pickup. That **** thing doesn't gain more that 5-10 regardless of the conditions. Maybe I'll slap a Chevy truck radiator on the front.....