Just to ensure that everyone here is on the same page, our brother Frank here lives in Germany where he sometimes rides his LT-500R on the world famous Autobahn. Sustained high speed durability is what he is looking for in his ride.

In addition to the engine mods, I believe 15 / 38 sprockets might be beneficial to lower the RPM's. Perhaps even an '87 5th gear change would work as well.