Try a dustbin fairing like what is at airtech.
http://www.airtech-streamlining.com/...1965.htm#price
Of course you'll have to mount your bars lower, mess with the headlight, etc., etc..
Other more normal things to do:
1) Use a good synthetic grease, and or oil for the shaft.
2) Get the tallest rear tire you can find (for the next one).
3) If you want to really shoot for the moon, explore second member units from the Shadow 750 or 1100, others of this engine family to slow the engine speed. The second member is where the output of trans takes a right angle via gears to the shaft where the countershaft would normally be. Machining may be required.
4) Lastly, use an induction timing strobe light to check to make sure all 4 plugs (dual plug head) are firing. The Transalp had an ignition module that was prone to fail causing only a single plug to fire in each cylinder.
Always liked these 500's, good score. I think the Cycle World test in the day returned over 60mpg, you can do better.