THE CLASSIC TRIMMER BIKE WHRRE YOU FRICTION DRIVE THE FRONT WHEEL WITH A FOOT PEG HAS A BALANCE PROBLEM
THE MOTOR HANGS OFF TO ONE SIDE LIKE A NUTSACK

PIC IS A REAR WHEEL DRIVE
DOESN'T THAT LOOK STABLE?

IF YOU'RE MOUNTING IT OVER THE FRONT WHEEL ON A SWINGING U-BRACKET SO YOU COULD PULL IT OFF THE WHEEL AS A CLUTCH,
(WHICH IS THE RIGHT CONFIGURATION FOR IT BY THE WAY)
YOU COULD POSITION IT WITH ITS CENTER OF MASS OVER THE CENTERLINE OF THE BIKE, AND THEN HAVE THE BMX PEG FRICTION DRIVE RIDING INSIDE THE U-BRACKET UNDER IT.
USE A PAIR OF SPROCKETS AND A CHAIN BETWEEN THEM

IT'S PARTS YOU'LL ALREADY HAVE AND IT'S NOT MUCH EXTRA WEIGHT
BESIDES IT'S WEIGHT OVER THE DRIVEN WHEEL SO IT'S EXTRA GRIP BOTH BETWEEN THE MOTOR AND WHEEL AND BETWEEN WHEEL AND GROUND