I am not aware of one. You may find something on Ebay which meets your spec, but a visible timer needs to be a bit bigger than a large watch.
The nearest I can get with a front winder is an Elgin Elapsed Time cockpit clock, but they are unlikely to be waterproof or fixable on your bars.

As an alternative, you might be able to adapt some type of large digital dive watch or laptimer, but I cannot imagine you wanting to use the pushers at 100 mph! On the other hand, your pillion passenger might be brave enough!