It is more than a set of cables. The whole shifting and throttle set up are different for the tiller motor. That drive shaft is expensive because they break a lot.
The 150 is a 5 cylinder with 3 carbs and 1 of the carbs feeds only 1 cylinder. Complete oddball made for around 2 yrs. The 55 hp uses the lower unit off of the smaller engines a known problem with those.