Very new to motors in general, just hoping to get some help.

[AYoungs]

I recently got a 1990 Evinrude Outboard. Like the title states, I am very new to motors and dont fully understand how they work. I bought a tiller handle from a guy who said it was 20-30HP, but a marina in town said it was a 9.9 and that it wouldnt work. What happens when you use a tiller that isnt the same HP as the motor? And do i need a wiring harness for a tiller motor? Thanks in advance!