The problem is that it loads the model from the mod, I wouldn't be able to change it unless I uploaded the whole mod to the workshop with the changes. That may happen after the next update for Synergy, if the mod developer would want to as well.
There is also a possibility that it will be compatible with the next version.

So maybe it won't crash?

Could you make it so it uses the vanilla zombie skin?

You don't actually require the patch to play it in Synergy, but I will add it anyways.
As for the zombies, Synergy crashes when it tries to load the default zombie. Some sort of configuration with alternate legs/arms/etc, it was a while ago. If you want to see, just try to spawn one in Lost Under The Snow.
ent_create npc_zombie

Also, why'd you change all the zombies to fast zombies? Kind of a cheap way to enhance the difficulty if you ask me.....

Hey, you should update the description to let people know that they have to download the source sdk patch. http://www.moddb.com/mods/lostunderthesnow/downloads/lost-under-the-snow-patch-source-sdk-2013

but underated.

great map