I have a starlane GPS 3x on my track bike, so long as you load the gps position of the start line it does the rest for you. you can also load intermediates is it will tell you if you are faster in each sector.
It has a great feature that you download the laps and compare them against each other.
They are over £300 but there is a guy in germany lists them on ebay with buy it now or offer and i got mine for £262 delivered