The width will depend on the spindle and cranks. If your engine kit did not come with a wide spindle then I would keep searching for your 1 piece. My 4 stroke kit came with a wide spindle and all I needed was the conversion kit. There is no plastic in the conversion kit. Aluminum adapters and steel cups and bearings.
I am another user of the short crank...but I just pedal from a standing start. I use it to keep my knees out of my face. I used to use the wide crank, but with my low seat (it's all about style), I needed the short crank.