There is not a kit for such a thing. I have heard that the Yamaha DT175 engine will bolt right in but I have not tried it so I have no way of knowing for sure. The DT engine is kick start and manual clutch. It comes from a dirt bike so I would think the kick starter rotates backwards which would cause it to hit the tire. Might be something to look into though if you really want to go away from the pull start.
So unless this is your dream trike tha you are going to hold on to forever then I would not bother.
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110