Have you priced a complete gearset?
You are going to be shocked.
It appears that Evinrude has discontinued this gearset (maybe), but it can be had aftermarket.
Your best bet will be to find a good used LU.
You can call Joe Eby at Advanced Marine in Interlochen MI. 231-275-5859. He can advise very well.
Also....noticing how you are referring to the parts...."smaller gear on the vertical shaft"....no offence, but it sounds like the job is going to be a challenge to you. Shimming, pulling the propshaft carrier, torqueing the pinion nut, various special tools, tough job for a newbie.
At any rate, good luck.