The real risk in creating races is that it's so easy to make them overpowered. Look at p. 282 of the DMG, and the "monster races" there -- none of them measure up to what the PC races have, and that's a good thing. To be fair to the other players, though, you need not try to replicate the MM stats, but use the MM stats to inspire something balanced with other PC races.
If I were making a Yuan-ti race, it would be Pureblood only. Given that what we see in the MM has 9d8 hit points. I'd suggest:
+2 Charisma
60' darkvision
Language: Abyssal, common.
Innate Spellcasting:
* speak with animals cantrip (snakes only)
* at 3rd level: poison spray cantrip
* at 5th level: suggestion
All that seems fairly straightforward, and just by itself makes a pretty powerful race.
Then there are the things you might want to add to more closely match the MM description:
* poison immunity: this is more powerful than Dwarven resistance.
* magic resistance
* +1 Intelligence
Let's start with the +1 Int -- it's implied by the stat descriptions, and would perhaps be worth including in exchange for the 3/day uses on poison spray and suggestion (which also then more closely matches the MM description; the limit is a bigger hit to PCs than monsters).
Poison immunity and magic resistance are both big things, and are (I'd suggest) why the Pureblood isn't listed on p. 282. But we don't need to let that stop us. I'd suggest giving the player a version of Dwarven Resistance, and allow it to be upgraded with a feat:
That would then give:
Yuan-Ti Pureblood.
+2 Charisma, +1 Int
60' darkvision
Yuan-ti resistance [=dwarven resistance]
Language: Abyssal, common.
Innate Spellcasting:
* speak with animals cantrip (snakes only)
* at 3rd level: poison spray cantrip (3/day)
* at 5th level: suggestion (3/day)
for starting characters, and then a new feat (which is only available at level 4 at the earliest):
Pureblood [feat]
You have developed innate abilities of your race.
* your dwarven resistance is upgraded to damage immunity: poison.
* you have the condition immunity: poison.
* you have magic resistance: advantage on saving throws against spells and other magical effects.
Now, I know you said Malison and not Pureblood -- that involves natural shapeshifting, etc. Is it the appearance that the player wants (i.e. to look like the dude on MM p. 309?). If so, the above would still work -- appearance is independent. If it's the shapeshifting and multiple forms, then you're going way beyond the abilities of other PC races, IMO
EDIT p.s. welcome to the boards!