Possibly could have been better if a proper spur gear was mounted onto the propshaft, allowing the motor to stay fully inside the chassis and the change of a standard pinion.The hardest part would be machining a spur gear adaptor which fits onto the propshaft, for sure! Even if the propshaft was bigger and a pin was secured into it to somehow hold the spur gear you've instantly increased rotating mass.
I think the propshaft may also need some retainer to keep wobble to a minimum near the new location of the spur gear too, like the original mounting has but I guess that would only be apparent on a testing session
Nice idea, but not very practical.