Throttle Jockey

My son started racing MX this year in the youth class on a Raptor 125cc. He is already in need of a bigger bike. I would like to build a hybrid 250 for my sons next step in ATV racing. I was looking at installing a yz250f motor in a yzf450. Has anyone done this? if so how did you solve the starting problem. It looks like the kick starter will hit the plastic Nerf bar. Are there any recommendations or alternatives I should consider?