(this might get a little convoluted but bear with me here)
If it's a pre-1994 1/2 then you can add a couple of horses, but you can't make it a 15.
1986 thru 1994 1/2, 8 horse models shared a powerhead with the 6 and 9.9 horse models utilizing the 12.6 cubic inch "split block" aka jugghead designed and built by Yamaha exclusively for use on Merc's.
Essentially the only difference is the carb and the exhaust plate - so if it breathes in and out "better" the 8 becomes a 9.9
The 15 horse has always used the 16.0 cubic inch block and that added displacement is absolutely required to produce the 15 horses.
But to answer your question about the 15 horse carb - Yes you can bolt it on and it will fit and if you replace the exhaust plate with one from a 15 as well you will get it to run without flooding most of the time, but it will not produce 15 horse AND it will burn significantly more gas than a 9.9 - I tried it as an experiment and in a head to head (almost) between the "franken 15/9.9" and a "real" 9.9, the 9.9 actually outperformed.
(*almost in that I ran the boat with hopped up motor through a few paces then back to the dock, strapped on the real 9.9 and did it again).
In mid-1994 Merc moved the 9.9 to the 16 cube platform which had been exclusive to the 15 horse since it's (modern) inception in 1987.
They also changed the exhaust plate and water tube on all the portable models (6/8/9.9/15).
So, if you want to convert it to a 9.9, regardless of what model year it is you will absolutely have to replace the exhaust plate and since the pre-94 1/2 plates are NLA you also have to replace the water tube.
That creates a small issue if you have the earlier model in that the original water tube was just a plain old straight tube that ran from the lower unit to the front of the exhaust plate where it was fastened with a (collar) and two screws - passing through a hole in the casting of the exhaust housing about half way down the "leg".
The new style water tube is simply pressed into a grommet and has a flair about 1/2 way down that is designed to stop the tube from falling out/getting pulled out when you remove the lower unit by causing that flair (bulge in the tube) to butt up against the hole through the casting in the leg (confused yet??) - the issue is, in the earlier models the hole in the casting is a little higher than the hole in the casting in the later models.
So if you have the earlier version you need to take a long drill (that will reach at least half way down the exhaust housing) and drill out the hole a little larger to allow the flair in the water tube to pass through (what I have done to help keep the water tube from falling out, which is a pain to try and re-insert with the leg in place, is to put a grommet around the water tube "above" the hole in the casting, essentially doing the same job as the flair was designed to do.
Once you have the exhaust plate sorted out, then you only need replace the 8 horse carb with ANY of the 9.9 carbs produced between 1986 and 2012 (the 9.9 was still being produced for export from the US until 2012).
There is about 2 dozen different model numbers, mostly all the same save the last few characters (eg. 1392-8846A24 replaced 1392-8846-A8 for instance)
Now, would it be worth the money?
If your current carb is salvageable only you can judge.
Since the line has been discontinued the cost of a new carb from Merc has jumped to almost 350 bucks (although you may find a used one, there are thousands of these motors out there).
The exhaust plate, water tube and seal (if required) goes between about 60 and 130 bucks depending on whether it's a short or long shaft.
So it could cost close to 500 bucks to convert an 8 to a 9.9 if using new OEM parts - is it worth maybe 250 bucks a horse??
I can't answer that for you - but it can be done.
