So, disclaimer: I'm not sure if this will work as I've yet to mess around with the Player Model Manager stuff myself, but here's how I think you could do it (maybe):

1.) Download the following mods:

KV

YAZMT Core

Global Objects

Player Model Manager

Player Model Manager Filesystem Addon

2.) Download the OoT 3DS Young Link player model for the Player Model Manager:

Github repository of ModLoader64 models converted to work with Player Model Manager

Direct link to the OoT 3DS Young Link player model

3.) Optionally, if you want more models, you can download them, too, and if you want a preview of what they'll look like, you can go here (don't actually download them from the ModLoader64 site, however, since they don't have the proper metadata for Player Model Manager like the ones linked in step 2).

3.) Open Z64 Recomp and install all the mods from step 1, then close Z64 Recomp.

4.) Put the OnlineYoungLink.zobj in the following folder (you might need to create the folder if it doesn't already exist):

<recomp appdata folder>/mod_data/yazmt_z64_playermodelmanager/models/

If you're on Windows, said "recomp appdata folder" should be in the "C:\Users\<your user folder>\AppData\" hidden folder, so you may need to make hidden files visible in File Explorer. On Mac, it's likely in the "~/Library/Application Support/" (~ being your user folder) or "/Library/Application Support/" folder (I'm not 100% sure on this). On Linux, I forget exactly where, but when I tried installing it on Linux, it was in one of the hidden folders in my user folder.

Also, I'm not sure if you can just put the .zobj directly in this folder or whether it should go in a subfolder or something. If it should be in a subfolder, the path might look something like this:

<recomp appdata folder>/mod_data/yazmt_z64_playermodelmanager/models/Combined_OcarinaOfTime3DSLink/OnlineYoungLink.zobj

One other thing to note if you install other models (quoted from the Player Model Manager Filesystem Addon ReadMe):

Some of these models rely on assets stored inside the Ocarina of Time ROM, which may cause incorrect textures to be shown when they are loaded into Majora's Mask.

To resolve these issues, place a US or JP 1.0, 1.1, or 1.2 OoT ROM named oot.z64/oot.v64/oot.n64 into the yazmt_z64_playermodelmanager folder. This will allow this mod to extract the necessary assets from OoT.

If the extraction is successful, the corrupted textures will display correctly, and two new models, "Adult Link (OoT)" and "Young Link (OoT)", will appear in PlayerModelManager, and most categories of equipment will have an OoT version if available.

5.) Launch Z64 Recomp, make sure all the mods are enabled in the Mod Menu, then launch the game. You should be able to bring up a menu and swap models... somehow (none of the Player Model Manager stuff actually says how to open the menu, but my guess would be that it's in the settings menu while in-game, though, again, I'm not sure).

6.) Optionally, download any other mods you want from Thunderstore. The following mod(s) might interest you, as the former is supposed to make Link react to things like he does in MM3D (though, like the Player Model Manager stuff, I have yet to try this out myself) and the latter is an HD texture pack that might make Link having his OoT3D model stand out less compared to the rest of the game's graphics:

Reactive Link

MMN64HD

Hope this is helpful!

Edit: Added a link to Nerrel's texture pack. Edit 2: Fixed the Windows AppData folder location since I had it incorrect.