LOL, Roadkill the "reins" from right-side handlebar to seatpost is only when parking. It keeps the left-heavy front fork from cocking and tipping the bike when parked. After "unhitching" the twin-engine monsters I wrap the "reins" around the HT gas tank's filler neck and away I go.
I presume that a front-mounted engine would blow less fumes directly to the rear than a rear-mounted or frame-mounted motorized bike. I really don't know.
For me, I have a spare front-mounted electric hub from an earlier project. I'd use that to pull a kiddie trailer.