Are you building a lap timer to work out speed or a lap counter to count 'the field'?
In my experience, some days you can have a lot of people 'fall down' and miss a lap or two
I think you will need to do some math and work out 'average' bike length?
Wouldn't you get multiple interupts from leading edge of front tyre, trailing edge, frame (including rider, etc), rear wheel; before you get next 'clean' signal when bike has past sensor?
Setting upon the slowest corner would also give any sensor more time to react, (rather than being fet up on the start finish straight)
Quickest and cheapest way to find out how fast MX bike is going is get a bicycle speedometer fitted to it 