I had one I just sold about a week ago. I wish I had known you were looking.
I believe 5Kids or 5J has built their own. One of them should be able to supply the list of items needed and a diagram. Could be a great project for you.
Haha.... you are giving me way too much credit with electrical things Evolution! Mechanical yes, electrical not so much... It was 5J's who made his own starting gate and timer. I am sure if you contact him he would help you with the plans.
I am a big fan of the The Judge solenoid remote starters.
I have the info from 5j but I would rather just buy one. I don't have time to build cars so no way I can build a start gate. I was going to buy a new one from jewkes but they have a month to 45 day back order.