Curt is correct, the parts will be scarce. Converting an old tiller to remote is not difficult from the aspect of controlling steering, but the throttle/shift linkage is where the scarce parts come in to play.
Generally, most will advise unless you can find a donor remote control motor, the juice is not worth the squeeze, meaning you're gonna struggle to find parts, the economics of the cost, the work to retrofit the motor (unless you're a marine mechanic) my recommendation would be either:
- Stay with the tiller, if it ain't broke don't fix it.
- eBay/Craigs List the old motor and buy a new Tohatsu (or other, but Merc's are made by Tohatsu as are Nisan) remote steer capable
If you go with #2, that leaves only acquiring the steering wheel, the control cables, the shift/throttle pod, and fabricating the console and side mount for the shift/throttle pod.
I have a similar pair of boats, and I chose to stay tiller, even though I have a complete shop it came down to not wanting to waist the space the console consumed, they simply get in the way on a small boat.
Probably not the kind words you wanted to hear, but been there, didn't do it. Just wasn't worth the effort.