As you noticed, my Motobecanes do ok. All I have done with them is to increase air flow by increasing the size of the air filter, then reduce the main jet size as much as I could without leaning out too much. I have also made sure the ignition timing is correct.
I don't know what your air box looks like, but I cut out 2 sides of mine, then used 2 squares of "UNI" filter foam, slit along all 4 edges, to fill the holes.
I know when my peds max out, they are spitting gas/oil back out of the carbs. I assume this is because the stock exhaust back pressure is causing reverse flow through the cylinder when the ports are open. I would expect your exhaust to reduce or eliminate this problem. The only other non invasive mod I can think of to increase your top end would be a velocity stack on the carburetor. I don't know if your air box would allow this, but maybe you could run without the air box just for the race. The principle behind the velocity stack is that the air mass in the stack is accellerated during the intake stroke and as intake vacuum declines, the inertia of the intake air, traveling at high speed, continues to stuff mixture into the crankcase, giving a slight "supercharging" effect.
I am not radical enough to try this but if you want to, I'm sure that someone here could come up with some approximate dimensions.
The only other ways to prep for the race are a complete tune up, including de-carbonizing the cylinder and cleaning the exhaust, check and lube all your bearings, run hard tires and make sure your brakes are not binding.
Oh, streched out horizontal, wearing only bathing trunks worke for Rolee Freigh (sp) at Bonneville when he broke the production motorcycle speed record. :-)